Add support for Ruby 2.1

:information_desk_person: These changes originally started out with the intent of adding Ruby 2.1 to the Travis CI build matrix, as well as to the list of supported Ruby versions. However, as changes have occurred over time in the gem dependencies, ActiveSupport and ShouldaMatchers, some additional updates were required to the test helper in order for them to be useable across the various Ruby versions. I've also updated the Rubinius version used in the build matrix to match Travis CI. :sweat_smile: