Change stdlib url in .fixtures.yml.erb

Switched to the HTTP url for cloning puppet-stdlib. I know it a tiny bit slower, but it works behind an HTTP proxy so is more useful for many in a corporate environment
Merge pull request #44 from chriscowley/patch-1

Change stdlib url in .fixtures.yml.erb
use the beaker environment variables rather than the old format
We're running the metadata checks so needed in the travis test group