Replace :rubygems in Gemfile with 'https://rubygems.org'.

Bundler some time ago started display warning message like follow:

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.

So, this change is to remove it and let everything go smother.