Reorganize unit tests, part I

* Move spec/shoulda to spec/unit_tests/shoulda
* Move spec/support/*.rb to spec/support/unit_tests/{helpers,matchers}
* Move spec_helper.rb to unit_spec_helper.rb
Reorganize unit tests, part II

* Change 'spec' Rake task to 'spec:unit'
* Require unit_spec_helper.rb in unit tests, not spec_helper.rb
* Re-namespace files in spec/support/unit under UnitTests
* Files in spec/support/unit/helpers no longer automatically add
  themselves to RSpec - this happens in unit_spec_helper.rb
* Extract RecordWithDifferentErrorAttributeBuilder and
  RecordValidatingConfirmationBuilder to separate files
Add new RSpec acceptance tests to replace Cucumber
Remove Cucumber tests
Fix unit tests to use new acceptance stuff
Tell Travis how now to run the tests
Add Ruby 2.1.4 to Travis
Fix deprecation warning on Rails 3.2
Fix association matchers + namespaced class_name

Fix `class_name` qualifier for association matchers so that if the
model being referenced is namespaced, the matcher will correctly resolve
the class before checking it against the association's `class_name`.

Take these models for instance:

module Models
  class Friend < ActiveRecord::Base
  end

  class User < ActiveRecord::Base
    has_many :friends, class_name: 'Friend'
  end
end

Here, the `has_many` is referring to Models::Friend, not just Friend.
Previously in order to test the association, you had to write:

    describe Models::User do
      it { should have_many(:friends).class_name('Models::Friend') }
    end

Now, `have_many` will attempt to resolve the string given to
`class_name` within the context of the namespace first before treating
it as a reference to a global constant. This means you can now write
this:

    describe Models::User do
      it { should have_many(:friends).class_name('Friend') }
    end