Update version in README

You guys missed the version update for the gemfile entry.

It should never be the case that `url` returns `nil`. If there is no url
that can be reasonable achieved, as is the case when the record the
attachment is attached to is unpersisted, then `url` should return the
default url.

For some reason the test in this PR would make all other ones blow up.
For reasons not fully understood at the time of writing, the alternate
spec I'd written doesn't. So, we'll replace that test with this one and
it'll be green.

[ci skip]

We upgraded the minimum required Ruby version to not include end-of-life
Rubies, but forgot to take into account that it's a backwards
incompatible change.

In this commit we rollback that change, allowing paperclip ~> 4.2.4 to
run on 1.9.2 again.

[fixes #1902]

A number of people were reporting that there were issues with the gem
dependency specifications changes we made recently. This will revert
them.

[ci skip]

Change it from 3.2.15 to 3.2.0. We'll let bundler handle installing the
latest version. With `~>` the last number doesn't really matter anyway.

A number of people were reporting that there were issues with the gem
dependency specifications changes we made recently. This will revert
them.

For the full details of what is included in this release, check out NEWS.

[fixes #1875]

Thanks to MORI Shingo of DeNA Co., Ltd. for reporting this.

There is an issue where if an HTML file is uploaded with a .html
extension, but the content type is listed as being `image/jpeg`, this
will bypass a validation checking for images. But it will also pass the
spoof check, because a file named .html and containing actual HTML
passes the spoof check.

This change makes it so that we also check the supplied content type. So
even if the file contains HTML and ends with .html, it doesn't match the
content type of `image/jpeg` and so it fails.

Mime-types 2.6.1+ has a columnar store that only loads the data that is
needed for a drastically reduced memory footprint.
cc/ @jeremyevans @halostatue

See: https://github.com/mime-types/ruby-mime-types/pull/96.

[fixes #1873]

* Add clarifications on use of helper methods
* Camelize 'ImageMagick'

[fixes #1874]

* Add note on new idiom for migration methods
* Mostly, correct punctuation and spelling
* Standardize capitalizations/upcase e.g., mime to MIME, paperclip to Paperclip

Removes RUNNING_TESTS file, that linked to CONTRIBUTING.

consistent with public URLs, this also changes the default behavior
for expiring URLs to use the https scheme rather than http.

Ensure that the correct gemset is included when running tests. Without this on my machine the  command would simply exit with no output

[closes #1655]

Normally, all of the settings would be set on `Attachment`, and they
were `deep_merge`d fine. However, if you set the global

    config.paperclip_defaults = { :validate_media_type => false }

then the `HasAttachedFile` class wouldn't pick up on it, because it only
uses things that are defined in the class. This change makes it so that
the default options are merged in to the ones that `HasAttachedFile`
uses when it sets up the attachment.

Fixes #1857

Present methods in roughly the same order as they are referenced,
to make it more graceful to read without jumping back and forth.

In other parts of the codebase, "filename" refers to the part after the
directory path it is contained in. e.g. `original_filename` in
attachment.rb.

This makes this file easier to read by conveying the range of arguments
ContentTypeDetector.new accepts.

Updated the `build missing styles` Capistrano task