Sensu check script for pp-smokey output

This script is run with a pp-smokey test name, ie `backdrop_read`. It
downloads the latest pp-smokey results from Jenkins and parses the
output. It returns an exit code which a Sensu check can interpret.
Add sensu::checks for each pp-smokey feature

These run on the monitoring box invoke the smoke test checker script
every minute.

https://www.pivotaltracker.com/story/show/67409248

[Delivers #67409248]
Add tests for JSON parsing script

Simple tests which capture the format of the JSON we're expecting and
demonstrate a success and failure case.

Also run them from Travis / run_tests.sh.
Merge pull request #386 from alphagov/sensu-checks-for-smokey-output

Add sensu::check for pp-smokey tests in Jenkins