Support multiple versions of rails

I want to use this gem in Rails 3.2 and 4.1 application.

I add config for travis-ci to test against multiple versions of rails, for 3.2, 4.0, 4.1 and change gemspec rails version :star: 

Could you please setup travis-ci for remote_factory_girl_home_rails :question: 
No, [travis already set `RAILS_ENV=test`](http://docs.travis-ci.com/user/ci-environment/#Environment-variables) in build environment.