Depend on our patched sensu-puppet

There was an issue in check creation in the official sensu-puppet
module, so I have fixed it and switched to using this until it is merged
upstream.
Expose warning and critical levels for 5xx and 4xx

Puppet didn't like the vars 5xx_* or 4xx_* hence five_* and four_*.
Merge pull request #51 from alphagov/http-check-fixes

Http check fixes