add _gat._anonymizeIp to google analytics config

This settings is to enable google analytics to anonymize IPs.

https://developers.google.com/analytics/devguides/collection/gajs/methods/gaJSApi_gat#_gat._anonymizelp
@peterflynn I haven't found any information about the order of argument appearance, except that the first argument  must be the Web-Property-ID. Everything else wasn't specifically mentioned in the docs.