Allows Experiment#in? to be called on removed experiments

  * No longer raises an exception
  * Returns false

  * We have a new requirement to return experiment information for
    possibly-removed experiments to mobile devices who can be running
    ancient versions from our API. The old behavior made obtaining
    historical information difficult.
Merge pull request #8 from howaboutwe/experiment_in_returns_false_for_removed_experiments

Allows Experiment#in? to be called on removed experiments