Expose a method to get kpi from service directly

Instead of getting the Dict and then getting the kpi
Use latest_kpi_for instead of accessing dict
Merge branch 'master' into dont-blow-up-when-no-kpi
Filter out services with no kpi
Rename method for clarity
Merge pull request #35 from alphagov/dont-blow-up-when-no-kpi

Dont blow up when no kpi