Fix purge method receiver regression

Hey folks,

#27 introduced a regression by erroneously sending the ```#purge_by_key``` method as class-level to ```FastlyRails::Client```. In reality, the spec provided with this change was actually testing sending them to an instance of the class. 

This fixes the receiver in both ActiveRecord and Mongoid. Regression originally discovered via spec suite at ```fastly/spree_fastly```. Only exists in 0.1.6 and not 0.1.5, obviously. 

Ran the test suite, it passes. But having said that, it was testing the correct receiver to begin with.