.travis.yml: test docs/ & update RTFD.org

It seems that unlike with Limnoria-doc, this doesn't prevent the build, but there are still errors.

```
Warning, treated as error:
/home/travis/build/Mkaysi/limnoria/docs/ADVANCED_PLUGIN_CONFIG.rst:43: ERROR: Unexpected indentation.
```

Oh, this is probably my line which should be easy to fix.