Use correct Rails module to determine Rails version

Ruby: MRI 1.9.3-p374
Rails: 3.2.11
I was getting the following error after adding the gem, bundle install and running `rails s`:

```
/Users/ian/.rbenv/versions/1.9.3-p374/lib/ruby/gems/1.9.1/gems/angular-ui-rails-0.4.0/lib/angular-ui-rails.rb:7:in `<class:Engine>': undefined method `version' for AngularUI::Rails:Module (NoMethodError)
	from /Users/ian/.rbenv/versions/1.9.3-p374/lib/ruby/gems/1.9.1/gems/angular-ui-rails-0.4.0/lib/angular-ui-rails.rb:5:in `<module:Rails>'
...snip...
```