Add delta parameter to statsdClient.Guage()

Add a third paramter to stastdClient.Guage() following changes to the upstream package[1].

We don't yet specify the versions of the packages we are using, so fix this as a temporary measure.

Fixes this [CI error](https://travis-ci.org/alphagov/govuk_crawler_worker/jobs/34377785):

> third_party/src/github.com/alphagov/govuk_crawler_worker/util/util.go:99: not enough arguments in call to statsdClient.Gauge

[1]:
https://github.com/quipo/statsd/commit/9604ea718f11257fb7e18c2c57bbce29ba20831b