Remove rubygems.org from source because we get warning like

Warning: the gem 'xxxxxxxxx' was found in multiple sources.
Installed from: https://rubygems.org/
Also found in:
  * https://your.rubygems.org/
You should add a source requirement to restrict this gem to your preferred source.
For example:
    gem 'xxxxxxxx', :source => 'https://rubygems.org/'