Push to the correct "gh-pages" remote

The current behavior is to push to "origin" regardless of the actual remote that is configured for the "gh-pages" branch. This change will use the configured remote name, defaulting to "origin" if all else fails.

For example, consider the following:

```
$ git config --get branch.gh-pages.remote
upstream
```

Currently running `groc --github` would result in the documentation being pushed to `origin`. With this change the documentation will be pushed to the `upstream` remote.