The thread_safe gem is being deprecated and all its code has been merged
into the concurrent-ruby gem. The new class, Concurrent::Map, is exactly
the same as its predecessor except for fixes to two bugs discovered
during the merge.

The concurrent-ruby gem is a toolset containing many concurrency
utilities. Many of these utilities include runtime-specific
optimizations when possible. Rather than clutter the Rails codebase with
concurrency utilities separate from the core task, such tools can be
superseded by similar tools in the more specialized gem. This commit
replaces `ActiveSupport::Concurrency::Latch` with
`Concurrent::CountDownLatch`, which is functionally equivalent.

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.

Now the default is always true.

Users still can set it using config.i18n.enforce_available_locales.

The option enforce_available_locales is only available on latest
versions, so require the last available one which has the option +
other related fixes and should not have backward compatibility issues.

We made a change in #13213 that depends on a new file that's only in
minitest 5.1.0+, so the version should be updated.

security fix.

Include the tzinfo-data in the generated Gemfile on Windows.

This reverts commit c24528fb.

Keep in sync with 3-2-stable.

Summary of the changes:
 * Add thread_safe gem.
 * Use thread safe cache for digestor caching.
 * Replace manual synchronization with ThreadSafe::Cache in Relation::Delegation.
 * Replace @attribute_method_matchers_cache Hash with ThreadSafe::Cache.
 * Use TS::Cache to avoid the synchronisation overhead on listener retrieval.
 * Replace synchronisation with TS::Cache usage.
 * Use a preallocated array for performance/memory reasons.
 * Update the controllers cache to the new AS::Dependencies::ClassCache API.
   The original @controllers cache no longer makes much sense after @tenderlove's
   changes in 7b6bfe84 and f345e238.
 * Use TS::Cache in the connection pool to avoid locking overhead.
 * Use TS::Cache in ConnectionHandler.

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.

Multi_json also upgraded.