font change because of Chrome problems

Change font to `roboto` after problems with Chrome reported on ML
Also give more space under nav-menu (for heading and/or logo images)
Remove un-used dependency in package.json (a bit unrelated to the pr...)
@arian as I see it it's not needed. `site` is there to ensure the `header` bar has [the right content](https://github.com/mootools/website/blob/master/views/partials/header.jade#L4).