Remove dependency on easy_class_to_instance_method

The [easy_class_to_instance_method][1] gem depends on activesupport >= 3.0.0, which means that it is now pulling in the newly released activesupport 4.0. The resque-pagerduty gem is intended to be simple, lightweight, and flexible. We don't want to require a relatively heavyweight library like activesupport (especially a version that breaks compatibility with ruby 1.8.7), unless it provides necessary functionality that is too expensive to implement ourselves.

 [1]: https://github.com/ramontayag/easy_class_to_instance_method