Adds support to test against Rails 4 and 4.1

Just pass the Rails version you want to test with RAILS_VERSION and run:

## Rails 4.1.0 Example

    $ rm Gemfile.lock
    $ RAILS_VERSION=4.1.0.beta1 bundle install
    $ RAILS_VERSION=4.1.0.beta1 bundle exec rake
Uses xhr method to test ajax requests in request specs.

Since Rails 4.1 has build in xss forgery protection for ajax get requests, we have to use the right way to test them.
Building against Rails 4.1.0.beta1 on Travis CI.