Make Gemfile specific to Rails 3 and introduce Travis "gemfiles" list
Rename test/dummy to test/dummy-rails3
Create Rails 4 dummy app and Gemfile
Add Rails 4 tests to Travis-CI matrix
Implement Rails 4 support

* Move mass_assignment_authorizer inside "included" block
* Check for presence of "protected_attributes" gem
* Issue warning if mass assignment sanitization is not strict
* Add "protected_attributes" gem to Rails 4 Gemfile