Upgrade sensu to 0.12 and sensu puppet to 0.7.7

- Old versions: sensu 0.11, sensu puppet 0.7.5 (patched)
- With sensu puppet 0.7.5 we used to maintain a github repo because the
  ordering of puppet installations meant that sometimes files in
  /etc/sensu/conf.d/checks/*.json were not generated.
- Upgrade to 0.7.7 direct from puppetforge as above bug no longer appears
  to be the case.

I (@paulfurley) have confimed with @robyoung that for a freshly provisioned monitoring-1 box, the checks are installed into ``/etc/conf.d/checks/`` before the services are started.

Note that we did, however, discover [this bug](https://www.pivotaltracker.com/story/show/62625052) - the gist of which is that later provisioning brand new checks does not cause sensu to be restarted.