Replace active_support/all with explicit requires

This is related to my comment on #466. Instead of completely replacing ActiveSupport we could cut down how much is required by the whenever gem to make it more palatable to those people who are running outside of Rails. This has the advantage that no code is changed.

Like @yob I'm getting a couple of minitest related errors which I've fixed although it's possible/probable these will cause issues on 1.8 versions of Ruby (ree and jruby 1.8 included).