Add activemodel runtime dependency.

Draper requires activemodel in `lib/draper.rb` line 2-4 but does not declare it as a dependency. 

When using draper outside a full rails environment it fails with a LoadError:
```
draper/lib/draper.rb:2:in `require': cannot load such file -- active_model/naming (LoadError)
```

Draper tests are passing because development dependency `active_model_serializers` has `active_model` as a dependency.