Use rspec-fire 1.2+.
Revert "convert instance_double usage to double in specs"

This reverts commit d20c9f824ad2006b9b6f6d975f49d6685d3e2b24.

In rspec-fire 1.2+ there is `instance_double`.
Merge pull request #60 from seomoz/use_instance_double

Use instance double