Change configuration setup to allow gem to be left out of default group

[pjmorse]

Currently the suggested setup for GemFresh, as in the README, is to have an initializer (at config/initializers/gem_fresh.rb). Running the initializer requires the constant GemFresh be defined in the application, which is fine as long as the gem is in the default group.

However, if a developer wants to leave the gem out of the deployed application by putting the gem in the development or test groups (e.g. by putting it in those groups in the Gemfile), then the initializer will raise NameError: uninitialized constant GemFresh whenever the application starts up.