Move require for 'rails/version' to the file that needs it

The file that causes rails/version to be loaded is not the same file that uses it. This is not a problem *per se*, but when running a generator, the ```Rails::Application``` might not get loaded, causing the calls to ```Rails::Api.rails4?``` fail with:

```rails-api-0.2.0/lib/rails-api/action_controller/api.rb:149:in `<class:API>': uninitialized constant Rails::VERSION (NameError)
```