Add enum to the highlighted activerecord functions

This Pull Request adds the Rails 4.1 `enum` to the activerecord keywords, because it is not highlighted currently:

![bildschirmfoto 2014-08-18 um 10 12 13](https://cloud.githubusercontent.com/assets/5446019/3949037/6f112c24-26af-11e4-8f52-5070e73ff1e0.png)