Fix gitlab logo

[pgjones]

There is no square version in Font Awesome. Fixes
938aa3d

[choldgraf]

thanks so much for fixing this :-)

[pgjones]

Thanks

[bollwyvl]

Folks: I think the gitlab_url was missing from theme.conf (which i didn't even know existed until I started #293)

I'd get:

preparing documents... WARNING: unsupported theme option 'gitlab_url' given

Anyhow, adding it on 1ea56e4 made that go away, and adding a little on-brand sass #548 makes it looks like:

[choldgraf]

@bollwyvl thanks for catching this, it is much appreciated :-)

[cblegare]

The above mentionned commit 1ea56e4 is not merged :(