Add a check for SSL certificate expiry

See https://github.com/sensu/sensu-community-plugins/blob/6eff6cfea601b87176ff39ebffd5d8016bf10097/plugins/http/check-http.rb#L89-L93
Reduce our maximum retention down to 100 days
Merge pull request #472 from alphagov/feature/ssl-cert-check

Add a check for SSL certificate expiry
Merge pull request #473 from alphagov/less_retentive

Reduce our maximum retention down to 100 days