Add requirement for sensu checks

The mongo and postgresql sensu checks use a script, but they didn't
require it so it wasn't present.

Related to https://github.com/alphagov/pp-puppet/pull/327

See https://www.pivotaltracker.com/story/show/66160874 [#66160874]
Merge pull request #329 from alphagov/fix-sensu-checks

Add requirement for sensu checks