Replace deprecated :rubygems source

Fix deprecation warnings thrown by bundler:

  The source :rubygems is deprecated because HTTP requests are insecure.
  Please change your source to 'https://rubygems.org' if possible,
  or 'http://rubygems.org' if not.

Fixes: bug #1175767
Change-Id: Id577d1d4c057827ce0d53c9bbedc66a8d290f54b