Add sensu check for Stagecraft healthcheck

So that we actually get alerted if it goes down. Mostly adapted from
backdrop's sensu checks.

Status endpoint code in stagecraft:

- https://github.com/alphagov/stagecraft/pull/64
- https://github.com/alphagov/stagecraft/pull/90

See https://www.pivotaltracker.com/story/show/58614078 [#58614078]
Merge pull request #302 from alphagov/add-stagecraft-sensu-check

Add sensu check for Stagecraft healthcheck