Updated version of grunt-contrib-stylus dependency

Was getting an error with version 0.11.0 of grunt-contrib-stylus:

```
Running "stylus:compile" (stylus) task
>> Destination not written because compiled files were empty.
Warning: Cannot read property 'dest' of undefined Use --force to continue.

Aborted due to warnings.
```

Version 0.18.0 seems to resolve the problem.