Rails 4 integration and Travis matrix integration

In this PR:

1) Made the Gemfile configurable in terms of Rails version
2) Updated the Travis CI matrix to include Ruby 2.0 and Rails versions 3.1.x, 3.2.x, and 4.0.x
3) Moved the tests to the standard file location for Minitest tests
4) Updated the helper code to work with Rails 4 when generating select/option tags
5) Updated the test to accommodate changes in tag output between Rails 3 and Rails 4

Travis CI passes for all elements of the matrix