Rbenv fix

Fixes #23.

Ansible's built in ```gem``` command doesn't run in the context of ```rbenv```.

This task wraps the ```gem``` command in a new login shell to make sure the shims are loaded into path. I made this a separate role in case maintainers don't want to force everyone into using rbenv.

See: https://stackoverflow.com/questions/22115936/install-bundler-gem-using-ansible/22121802#22121802

```
- role: rbenv
```