Rails 4 compatibility

I'd like to apologize. Indenting the specs for `Twilio::RequestFilter` to cover both `.filter` and `.before` resulted in a fairly confusing diff. None of the specs were changed, otherwise.

This pull request:

* Lowers the required version of builder to `>= 3.1.0` rather than `>= 3.2.2`, solving a conflict with [actionpack](https://rubygems.org/gems/actionpack) 4.0.0 which specifies a dependency on builder `~> 3.1.0`.
* Silences a Rails 4.0.0 deprecation warning in `Twilio::RequestFilter`. Filter objects can now specify `#before`, `#after`, and `#around` methods when used in a controller. For now, an alias is used (it might be more suitable to switch the alias around, given `.filter` is the method that was deprecated).
* Railtie for `voice` responses is loaded in Rails 3.0.0 and up.

Anyway, let me know what your thoughts are on this. Thanks for maintaining this awesome gem!