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.

[1]:
https://github.com/quipo/statsd/commit/9604ea718f11257fb7e18c2c57bbce29ba20831b
Merge pull request #78 from alphagov/update-statsd-guage

Add delta parameter to statsdClient.Guage()