[ci skip]

Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

Rails 6 will only support Ruby >= 2.3.

This reverts commit 3420a145, reversing
changes made to afb66a5a.

".. with __dir__ we can restore order in the Universe." - by @fxn

Related to 5b8738c2

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

Resolves #24924.

This reverts commit 62aa850f, reversing
changes made to 8c1f248c.

There is no reason to disallow mail 2.5 so we don't need to bump the
version constraint since people are still able to use mail 2.6 and get
all the memory saving that was pointed in the pull request description.

Mails downstream dependency (Mime-types) has been shown to decrease
memory usage significantly in its 3.0 release. This memory decrease
will be a big win for users upgrading to Rails 5.

Lets nudge users to upgrade Mail alongside Rails.

and fix the grammar in the ruby_version_check.rb user message.

This is a follow-up to #19257

Stems from [this comment](https://github.com/rails/rails/pull/18203#issuecomment-68138096) by @robin850
and by the blog post http://weblog.rubyonrails.org/2014/12/19/Rails-4-2-final

[This article](http://weblog.rubyonrails.org/2014/8/20/Rails-4-2-beta1/#maintenance-consequences-and-rails-5-0) states that:

> Rails 5.0 is in most likelihood going to target Ruby 2.2.

Before the exact minimum version is fully decided, @arthurnn [suggests](https://github.com/rails/rails/pull/17830#issuecomment-64940383)
that **at least** version 2.1.0 **must** be required by the `gemspec` files.

This allows Rails users to install mail 2.6 which relaxes
the mime-types dependency, which is a big win for a lot of people.

Previously, the mail gem restricted mime-types to ~> 1.16
but now it has expanded to [">= 1.16", "< 3"]

And the mime-types maintainer will also be checking that
2.x releases don't break mail.

See https://github.com/mikel/mail/pull/713
https://rubygems.org/gems/mail/versions/2.6.0

This reverts commit c24528fb.

Obviated by rubygems/rubygems@486ed83cc8e706069213c5d406122f4cfcca9e7f

This reverts commit bb8923de.

RG2 packager expects each spec.files path to be a file and bombs when it tries to tarball a dir.

May revert if rubygems/rubygems#413 is accepted.

Organized the gemspec files a bit.

 * Made quotes more consistent (single quotes dominated, so I used
   that).
 * Moved license line down a line, separating it logically, and removed
   the extra whitespace before its = operator.
 * Minor whitespace fixes.

  Second security vulnerability found in mail file delivery method
  patched in version 2.4.4.

Increasing minimum version of mail due to security vulnerability found in Mail 2.4.1 for sendmail or exim