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-r
ails/

* 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

This method previously existed to hide the complexity of different
ActionView Template base classes in different rails versions. Since
that is no longer the case, it is simpler to remove it and use
ActionView::Template::Error.
Merge pull request #728 from teeparham/drop_rails_31

Drop support for Rails 3.0 and 3.1.