Backup credentials file for ruby .gem folder only

As described in #94, `.gem` folder holds configuration information necessary for publishing ruby gems, like the credentials needed to to upload to rubygems.org. But it also holds installed ruby gems and specs for these gems. IMO these should not be saved as they're not configuration information.

For example, my `.gem/ruby` folder is more than 1G in size (I'm a ruby developer, having multiple versions of ruby installed).

With this change, mackup will only backup `.gem/credentials`. If there're other configuration information files we could add them too.