Updated the `build missing styles` Capistrano task

Deprecation Warnings:

```
`failure_message_for_should` is deprecated. Use `failure_message` instead. Called from /Users/mugi/git/paperclip/spec/support/matchers/have_column.rb:11:in `block in <top (required)>'.
```

Prioritize MimeMagic over the `file` binary for content type detection
when it finds a match. Fall back to `file` if MimeMagic can't match
anything.

`file` incorrectly detects Open Office XML files (e.g., xlsx, docx) as
zip since they're implemented as zipped archives of xml files. MimeMagic
detects them properly.

[houndci](https://houndci.com) was try to add it because I thought set of tools that can be found in the local environment is want I 'm handy when you pull request

    bundle exec rake cucumber

to log

```
Capybara::RSpecMatchers::HaveText implements a legacy RSpec matcher
protocol. For the current protocol you should expose the failure messages
via the failure_message and failure_message_when_negated methods.
```

See
https://github.com/jnicklas/capybara/issues/1254
https://github.com/jnicklas/capybara/pull/1219

Use Updated Version of `aws-sdk` in Docs [ci skip]

The `aws-sdk` development dependency was updated to `~> 1.6` in #1816
to resolve issues with the creation of 0-byte files on S3 (see #1729).
However, there is no runtime dependency on `aws-sdk`, and the
documentation still suggests that users install an earlier version of
`aws-sdk` that produces the problems in the referenced issue.

This commit updates the docs to recommend the use of `aws-sdk` `~> 1.6`
with Paperclip.

* Uses Paperclip's Tempfile instead of Ruby's in Thumbnail
* Also includes a test for TempfileFactory to be sure it does not revert to similar behavior
* In the tests, give the OS some wiggle room in the Tempfile name to ensure the error does not occur before we are ready for it. The wiggle room is needed because the randomized part of the name has a variable number of characters.

Rescue only `NotIdentifiedByImageMagickError` in
`Attachment#post_process_style`. Other errors should bubble up, because
they are meaningless for the end-user, but important for the programmer.

Fixes #1352.

Remove RubyForge project name from gemspec

http://docs.travis-ci.com/user/workers/container-based-infrastructure/

With Container based CI, public projects can cache the results of bundle
install

It's useless because there is the same conditional in https://github.com/thoughtbot/paperclip/blob/master/lib/paperclip/railtie.rb#L25, and if ActiveRecord is not defined, Glue will not run.

Updated gemspec to fix issue #1729

Updated gemspec to require aws-sdk gem ~> 1.6

* Move the thanks to contributors to the contributing section
* Introduce the new "About thoughtbot" section.

The container infrastructure (beta) has many advantages:
* More memory
* More CPU
* builds start much faster.
* builds run much faster (usually)

However, `sudo` cannot be used when running in this environment (which
is enabled with the `sudo: false` configuration.

The reason behind this I'm trying to use StringioAdapter inside the mailer, and it fails because `length` used in one of the `mail` gem:
https://github.com/mikel/mail/blob/master/lib/mail/message.rb#L1997

Also you can see that normally `size` has `length` alias, for example in a `StringIO` class:
http://ruby-doc.org/stdlib-1.9.3/libdoc/stringio/rdoc/StringIO.html#method-i-length

Let me know, if this change requires test case, I will add it.

Turns out that aws-sdk v2 isn't backwards compatible in the slightest,
so we should cap that at v1. And cucumber 1.3.17 was giving issues with
ruby 2.2.0. So let's update those.