Drop support for Rails 3.0 and 3.1 (minimum is Rails 3.2)

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
[ ] remove from travis matrix
[ ] `InstallGenerator` has a code branch for copying CSS files that's no longer required