Fix detection of Rails::Engine for Rails 4

We tried to upgrade to angularjs-rails 1.2.12, but now get this error:
```
couldn't find file 'angular'
```
when trying to load the application.

Assumedly the culprit is this commit: 137f7cfffa9c5e9cd9dac9d9a3525d7738060612
We see that locally we get into the `elsif defined? Sprockets` condition, which is probably not what was intended for a Rails 4 project.

It works for us now if we use `defined? ::Rails::Engine`, but I'm not smart enough with engines to know exactly why or whether it will still work for Rails 3.