Builder dependency incompatible with Rails 4

[ewoodh2o]

The builder dependency in v0.3.12 was changed from >= 2.1.2 to ~> 2.1, >= 2.1.2 to remove the open-ended version. That's probably best practice, but now it conflicts with Rails 4.x since either actionpack (4.0) or actionview (4.1, 4.2) depends on builder ~> 3.1.

Most other gems seem to have left the open-ended dependency. Was there a specific issue that necessitated this change, or could we consider relaxing it back to >= 2.1.2?