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: I10227cd21b36f8dd7e575e5e630d3d4823333417