Update grammar to the latest ruby-slim.tmbundle

Updates the grammar to match:
https://github.com/slim-template/ruby-slim.tmbundle/tree/5fc32699981a24fcc7a256055b588516234bdfb8

Fixes #2. Screenshot:

![image](https://cloud.githubusercontent.com/assets/992008/4658823/8feb9366-5504-11e4-90ae-278554d807d9.png)