Add Ruby 2.1 and Rails 4.1 to test matrix

* Add Ruby 2.1 to travis configuration
* Update Rails 3.2 and 4.0 appraisals to latest versions
  * Add I18n.enforce_available_locales setting to test app to avoid
    deprecation warning.
* Add Rails 4.1.0.beta1 to appraisals
  * Test unit integration feature updated to account for differing test
    output under rails 4.1
* Update gems