Add movingAverage function to 4/5xxx checks

The checks are over the previous 60 seconds but only consider the most
recent point.
Relax 4/5xxx sensu check limits

The 4xx checks specifically are firing almost continuously.
Merge pull request #130 from alphagov/improve-4xx-checks

Improve 4xx checks
Merge branch 'master' into use-fqdn-for-graphite

Conflicts:
	modules/performanceplatform/manifests/proxy_vhost.pp