Use source parameter for chef_gem if it exists
Use source parameter for chef_gem if it exists

It seems as though using the options parameter on Chef 15 for rubygems_url isn't
working. Instead it should be using the source parameter if it exists.

This ensures that folks can install mixlib-install from whatever source they
want.

Signed-off-by: Lance Albertson <3161cdc7bf197e4c3a35b9cbe358c79910f27e90@osuosl.org>

chef-cookbooks/chef_client_updater,chef-cookbooks/chef_client_updater