Drop support for Rails < 3.2.13

I'd like Formtastic 3 to require a minimum of Rails 3.2. I've proposed a deprecation notice be added into Formtastic 2.3 as part of #948.

There may be a bunch of extra code we have in place to support 3.0 and 3.1, let's start a list:

- [x] remove from the Appraisals
- [x] bump actionpack dependency to 3.2
- [x] remove from travis matrix
- [x] `InstallGenerator` has a code branch for copying CSS files that's no longer required
- [ ] `Localizer.normalize_model_name` has some work-arounds for 3.0 only
- [x] `ButtonAction` has a work-around for 3.0 missing `button_tag` helper

There must be some more subtle ones… ideas?

*Update:* New minimum is Rails 3.2.13:

- [x] remove from the Appraisals
- [x] bump actionpack dependency to 3.2.13
- [x] remove from travis matrix
- [ ] merge this sucker in after 2.3.0 is released and we have a 2-3-stable branch