Rebuild Gemfile_master.lock based on the lastest Gemfile.lock
actionpack-action_caching was released a long time ago
Use Rails 4.2+ API for typecasting on master

Note: this is still considered a private (internal) API on Rails side and is
subject to change in the future.
Boolean -> String quoting has changed on Rails master

CustomField.create(name: 'zomg', value: true).reload.value # => 't' on Rails 4.1, '1' on 4.2

rails/rails@42be84ba
Avoid a deprecation warning by poly-filling #deliver_now and #deliver_now
HAX: force the lazy `MessageDelivery` object to create the mailer

Starting from Rails 4.2, calling MyMailer.some_method no longer result in an
immediate call to MyMailer#some_method. Instead, a "lazy proxy" is returned
(this is changed to support #deliver_later). As a quick hack to fix the test,
calling #message (or anything, really) would force the Mailer object to be
created and the method invoked.
`Sprockets::Context` takes a `Pathname` for its third argument

As far as I can tell, sprockets has always expected the third argument to be a
`Pathname` object. Not sure what actually changed and why it was working before,
but it now throws a `NoMethodError` for `dirname` on `String` in
`Context#resolve`

    # , not sure why this was working before
Use *_url instead of *_path to avoid a deprecation warning
HAX: check the `message` object, not the `MessageDelivery` object

See 669bf73 for background. It's probably better to rewrite these test without
using the internal `NullMail` class anyway.
Added back the `last_use` timestamp to the `AbstractAdapter`

This was removed in https://github.com/rails/rails/commit/9e457a8654fa89fe329719f88ae3679aefb21e56
Upgrade to the latest AMS 0.8.x (unreleased)

The current released version (0.8.2) does not work with Rails master at all. In
fact, it was quite surprising to me that this is the only test that broke...

See https://github.com/rails-api/active_model_serializers/pull/655
Add RAILS_MASTER to the travis build matrix