new rubygems source

"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."
use new guard API

https://github.com/guard/guard/wiki/Upgrading-to-Guard-2.0#changes-in-guardguard
Merge pull request #10 from jjb/guard2-api

Guard2 api