Ignore index name in `index_exists?` when not passed a name to check for

Remove legacy mysql adapter

Remove warnings from actioncable test

Require Nokogiri >= 1.6.7.1

Many CVEs released: https://groups.google.com/forum/#!topic/ruby-security-ann/aSbgDiwb24s

Only dup Ruby's Hash and Array.

When calling `to_h` on an `ActionController::Parameters` instance it would
`deep_dup` its internal parameters.

This inadvertently called `dup` on a passed Active Record model which would
create new models. Fix by only dupping Ruby's Arrays and Hashes.

Logging Action Cable to STDOUT caused the development log to see double
messages like this:

```
Started GET "/" for ::1 at 2015-12-17 15:21:34 -0500
Started GET "/" for ::1 at 2015-12-17 15:21:34 -0500
Processing by Rails::WelcomeController#index as HTML
Processing by Rails::WelcomeController#index as HTML
Rendered /welcome/index.html.erb (0.0ms)
Rendered /welcome/index.html.erb (0.0ms)
Completed 200 OK in 3ms (Views: 1.3ms | ActiveRecord: 0.0ms)
Completed 200 OK in 3ms (Views: 1.3ms | ActiveRecord: 0.0ms)
```

Now that Action Cable is part of Rails it doesn't need it's own logger
and will log to STDOUT via the local dev server here:
https://github.com/rails/rails/blob/master/railties/lib/rails/commands/server.rb

ApplicationMailer should be generated by default just like every other Application* parent

We call the thread variable accessors on `Thread.current`, which matches Ruby's 
documentation:

http://ruby-doc.org/core-2.2.0/Thread.html#method-i-thread_variable_get

Fix these to stay `current` ( ͡° ͜ʖ ͡°)

Expanding the documentation for collection=objects [ci-skip]

traditional make sense with `<tt>ActiveSupport::TaggedLogging</tt>`

Since the attributes API is new in Rails 5, we don't actually need to keep
the behavior of `attribute :point`, as it's not a breaking change.

Warnings coming from code and test are removed

Clarify thread_mattr_accessor subclass behavior documentation

Add actioncable to build step for release

[ci skip]

[ci skip]

Update USAGE [ci skip]

Fix typo

Generate API doc for actioncable

Fix typo in thread_mattr_accessor doco [ci skip]

Add actioncable to list of release frameworks [ci skip]