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.

Change-Id: I63d0497d96a210ac9b6e01a9f0bd4fb4b4f14d9f
Merge "Replace deprecated :rubygems source"