prevent multiple requires

Some gems like Devise are hooking into Rails reloading process and force loading of User **before** annotate models requires them (second time)

Another example is FactoryGirl, which hooks in railtie-initializer and loads constants from ```factory :some, class: SomeClass```

As such there are not only warnings about `already initialized constant` but also errors if User gets enum field from Rails 4.1, e.g

```
...app/models/user.rb:28: warning: already initialized constant ...
...app/models/user.rb:29: warning: already initialized constant ...
...app/models/user.rb:38: warning: previous definition of ... was here

Unable to annotate user.rb:
You tried to define an enum named "..." on the model "User",
but this will generate a instance method "...=", which is already defined by another enum.
```

and user **wont get annotated**.
This pull fixes it.