Merge branch 'release/2.3.1.0' into develop

* release/2.3.1.0:
  Bump version to 2.3.1.0
Test against Rails 4.0.0.beta1
Test against JRuby and MRI 2.0.0
Ruby 2.0.0 for Rails 3.1 is not supported
Rails 4.0 on JRuby is still wonky
Add test for #44
Make clearfix mixin available as a class (Closes #45)
Bump version to 2.3.1.1
Merge branch 'release/2.3.1.1'

* release/2.3.1.1:
  Bump version to 2.3.1.1
  Make clearfix mixin available as a class (Closes #45)
  Add test for #44
  Rails 4.0 on JRuby is still wonky
  Ruby 2.0.0 for Rails 3.1 is not supported
  Test against JRuby and MRI 2.0.0
  Test against Rails 4.0.0.beta1