Do not install ruby-debug on travis-ci.org, it only wastes time

See http://about.travis-ci.org/docs/user/languages/ruby/ (the "Exclude non-essential gems like ruby-debug" section) for reasoning.

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)