install ruby gems from https://rubygems.org instead of :rubygems

To resolve this warning:

    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.

Gemfile

@@ -1,4 +1,4 @@
-source :rubygems
+source 'https://rubygems.org'
 
 if puppetversion = ENV['PUPPET_GEM_VERSION']
   gem 'puppet', puppetversion, :require => false