(maint) Update gem source in Gemfile

Without this patch installed the following warning message is displayed
on a Debian 6 system when running `bundle install --path vendor/bundle`.

    The source :rubygems is deprecated because HTTP requests are insecure.
    Please change your source to 'https://rubygems.org' if possible

This patch addresses the problem by changing the gem source to
https://rubygems.org which avoids the warning.
Merge branch 'update_gemfile' into 1.6.x

* update_gemfile:
  (maint) Update gem source in Gemfile