Don't do a service get before purging
* adds purge_by_key method to FastlyRails::Client
* updates ActiveRecord and Mongoid purge methods
* tests
Reduce the coverage of stub_request
* In reality it's only used to stub api.fastly.com/login
* add stub_request for purge URL
Merge pull request #27 from fastly/no-service-get-in-purge

Purge without fetching the service first