(maint) Update rubygems source to use https

This gets ride of the warning with Bundler 1.3.0:

    The source :rubygems is deprecated because HTTP requests are
    insecure.
    Please change your source to 'https://rubygems.org' ...
Merge branch 'maint/2.7.x/gemfile_https' into 2.7.x

* maint/2.7.x/gemfile_https:
  (maint) Update rubygems source to use https
Merge branch '2.7.x' into 3.0.x

* 2.7.x:
  (maint) Update rubygems source to use https