Added FactoryGirl configuration details

Fix outdated method description in helper module

Fixes #2130.

link in Readme to source code of Paperclip::Processor

for better visibility and better User experience of documentation

Most contributors don't know to update the NEWS file when submitting
code.

Currently paperclip fails with `NoMethodError` on `nil` when file is not
present. Since `nil` is proper response from fog on missing files, and
this method already returns `false` if any error happen (see line 178),
returning `false` here would be properly handled on client code.

[closes #2173]

Update NEWS.

[ci skip]

Adds failing spec to command line dup check

[fixes #1670]
[fixes #2169]

[fixes #2164]

Bugfix: Dynamic fog directory

`Paperclip::Storage::Fog#host_name_for_directory` assumes a String-like
object is set and doesn't check if `@options[:fog_directory]` is
callable, while `Paperclip::Storage::Fog#directory` does. This extracts
a new method with the condition and refactors the other two methods to
call it.

Fixes #2018, #2093.

@melissawahnish noticed that while we are on beta v5 will not be found.

[ci skip]

This configuration constant was useful for supporting different versions
of AWS SDK. We don't any longer, and we can set the value directly.

[fixes #2148]

* `AWS::Core` is an AWS SDK v1 API, we can drop it
* We don't use the "I attach a" cucumber step

Brings back warning on not supported SDK versions.

Related: https://github.com/thoughtbot/paperclip/commit/25be0b25d67923e28419c6146e8f8307741a974b#commitcomment-16831800

We don't support AWS SDK v1 any longer.

Drops Rails 3 branches.

[fixes #2101]

Update example to add mime-types to file extensions

Remove Rails 3 snippets

Rails 3 is not supported anymore. We can safely remove mentions and
snippets for it.

Drop unused variable definition (`options`).

The old example gives deprecation warnings from recent 2.x versions of mime-types. As of f0bd25ad the Gemfile also allows version 3.0 or newer. The new example should work regardless of the version of the mime-types gem. [skip ci]

`>= 2.0.34` is the proper restriction. We weren't allowing `2.0.33`
before.

Paperclip 5 no longer supports aws-sdk v1. Having a lot of constraints
in one gem dependency caused issues when pushing the gem (ref:
https://github.com/rubygems/rubygems/issues/1564).

This commit simplifies the dependency restriction to `>= 2.0.33`.

Pairing with @kitop

Update NEWS.

[fixes #2134]
[ci skip]

Add Rails master appraisals in preparation for Rails 5

While working on this branch, Kito found that this test fails due to:

https://github.com/rails/rails/commit/6ec8ba16d85d5feaccb993c9756c1edcbbf0ba13#diff-fdcf8b65b5fb954372c6fe1ddf284c78R76

We are not yet sure if it's a bug in paperclip or in Rails itself. With
current `ActiveModel::Errors` implementation the following happens:

```
record.errors # => @messages = {}
record.errors.include?(:foo) # => false
record.errors # => @messages = { :foo => [] }
```

Which bit us in:
https://github.com/thoughtbot/paperclip/blob/69f18375333234b6f395300266e2612936bd242e/lib/paperclip/validators/attachment_file_name_validator.rb#L23

Another related Rails commit:
https://github.com/rails/rails/commit/b97035df64f5b2f912425c4a7fcb6e6bb3ddab8d

I worked around the issue by changing what we assert in this spec. I am
still not sure that this is a bug in current Rails master.

cc @kitop for review

Ref: https://github.com/rails/rails/commit/481e49c64f790e46f4aff3ed539ed227d2eb46cb

`silence_stream` was deprecated in Rails 4.2 and removed in Rails 5
because it was not thread safe. Paperclip is not running the tests in
threaded mode, it's safe to add the method back in a test support
module.

Ref: https://github.com/rails/rails/commit/2386daabe7f8c979b453010dc0de3e1f6bbf859d

Specifically:

> Chains of callbacks defined **with** a `:terminator` option will
> maintain their existing behavior of halting as soon as a `before_`
> callback matches the terminator's expectation. For instance,
> ActiveModel's callbacks will still halt the chain when a `before_`
> callback returns `false`.

In terminator callbacks, the `result` value changed to be a lambda now.
This change reflects that, updating the terminator to compare the result
of the block to `false`.

Also:

* Removes Rails 4.1 branch (we don't support it anymore)
* Rewrite for legibility: change `unless` for `if`

`bundle exec rails -v` was taking seconds every time, making the test
suite run very slow. This changes the code to compare the version
against `ActiveRecord::VERSION`, which is going to be the same one as
`rails -v`, making it much, much faster.

Simplifies condition: we no longer support Rails < 4.2, so we test for
`>= "5.0"` or fallback to `4.2` branch.

- Also bump Travis Ruby 2.2.x version to 2.2.4

We found that uploading large files to S3 would result in a socket error
("connection reset by peer") occasionally and lately much more
consistently. In researching this I saw that many people got this error
when uploading too large of a file without multipart chunking. I would
have assumed fog did this automatically but the default chunk size may
be too high. In order to address this I wanted to drop the chunk size to
100MB.

Rather than hard-code this I opted to expose a `fog_option`
configuration option that lets me pass any additional options I want to
the fog's `#create` call. This is similar to the `fog_attributes` option
implemented in CarrierWave which [addresses the same
problem](http://stackoverflow.com/a/11867978/201911).

We've been running this now for a week in production and it seems to
resolve the issue.

https://github.com/thoughtbot/paperclip/pull/2135

Related with https://github.com/thoughtbot/paperclip/issues/2122

[ci skip]

See https://www.ruby-lang.org/en/news/2016/02/24/support-plan-of-ruby-2-0-0-and-2-1/

Remove attr_protected related specs (was removed in rails 4)

Skip calculating fingerprint when it is not used

The MD5 sum of an attachment is only necessary if the model has a corresponding
_fingerprint column. If this column is absent, there is no need to calculate the
MD5, which can be an expensive operation for large files.

Accomplish this by deferring the fingerprint calculation using a block. If the
_fingerprint column is absent, the block is never called, and the calculation
is avoided.

Modify the Gemfile for testing with mime-types 3