The methods failure_message_for_should and failure_message_for_should_not
have been updated to failure_message and failure_message_negated respectively.
Alias to the old methods to remain backwards compatibility with RSpec 2.
Rails 4 moved the layouts ivar from `@layouts` to `@_layouts`.
This change introduces what I have (for now) called the 'RailsShim'. The
idea is that this single class will house the rail version workarounds,
which can then be reused through the code. This paid off here because
both the spec and the implementation need access to the layouts ivar.