Officionally dro support to Ruby 1.8 and Rails 3.0 / 3.1
Only add protected_attributes to Rails 4 Gemfile, require it for test env
Handle text fields tests for both Rails 3 and Rails 4

The default size attribute is not generated for inputs on Rails 4
anymore, so we have to account that.
Remove old test code to support Rails 3.0
Remove old test code to support Ruby 1.8
Inherit from proper proxy/basic object to avoid deprecation warnings

Rails 4 deprecated BasicObject in favor of ProxyObject.
Remove support for passing extra options to assignment methods

Old mass assignment options like :without_protection are being killed in
Rails 4, so the 3.2.x compatibility is also dropping support for it in
favor of using things like strong parameters.