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.
Merge pull request #189 from alphagov/upgrade-sensu-puppet

Upgrade sensu to 0.12 and sensu puppet to 0.7.7