Make Gemfile work with newer versions of ruby

Puppet 3.1.0 wouldn't play nicely with my system ruby:
https://github.com/puppetlabs/puppet/pull/2184/files

This was fixed in newer versions of the puppet gem, so I've just updated the Gemfile to a newer version, with some logic to leave users of ruby < 2.1.0 on the puppet version that was there before.

I'll leave it to you guys to decide if you'd rather just bump everyone up, but I figured there was a reason for specifying ~>3.1.0