link "GITLAB" logo to root_url instead of '/'

If using multiple Rails apps in subfolders, the upper left "GITLAB" button links to '/' and thats kinda wrong.
This fix creates a link to the root_url instead, which works in every case.

Use HTTPS protocol in Gemfile

Don't install development and test gems in production

Switch to the https protocol in the Gemfile & Gemfile.lock since the
git protocol is sometimes blocked by firewalls (and is not easily proxied)