Update GoSquared w/ `identify` method

The GoSquared tracker now has nicer `identify` and `properties` functions to replace the visitorID and visitor setting. I believe it's all backwards and forwards compatible.

Note: any idea why https://github.com/segmentio/analytics.js-integrations/blob/master/lib/gosquared/index.js#L51-L54 is in there? It doesn't seem to be in any other integrations