Add activesupport dependency

https://github.com/kangguru/rack-google-analytics/blob/master/lib/rack-google-analytics.rb#L1 requires activesupport so we need to add it as a dependency.

Figured that, when trying to add it to octostalker https://github.com/arthurnn/octostalker/commit/9d8452adcf841e4fd87baf20e96368af9259680e . See that I had to add activesupport otherwise it wouldnt work.