Drop support for Rails 3.0 and 3.1.

Rails 3.0 and 3.1 are no longer supported, matching the official Maintenance Policy for Ruby on Rails.

See http://weblog.rubyonrails.org/2013/2/24/maintenance-policy-for-ruby-on-rails/

* Remove Rails 3.0 and 3.1 Gemfiles.
* Remove Rails 3.0 and 3.1 from the Travis build matrix.
* Require Rails >= 3.2 in the gemspec.
* The Haml::Plugin class does not need to inherit from anything.
* Remove Haml::Util.av_template_class