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.
Removes the feature to pass a layout via params.

The layout will always be the default layout from host Rails app.

If the request is an ajax request, then no layout will be used.
Disables the jasmine specs again on CI.

Since I can't find a reliable way to get correct exit codes for the rake tasks (because of simplecov?), we need to disable them again :(
Ignore .bundle folder.
Render the application layout in preview frame.
Uses simplecov 0.7.1

SimpleCov >= 0.8.1 has exit code issues: https://github.com/colszowka/simplecov/issues/269
Upgrades to ActsAsTaggableOn 3.0

Fixes issues with Rails 4.1
Allow Turbolinks >= 2.0
Require gems in engine, so things like rake tasks are available.