Updated rake spec to run with *all* adapters
Updated rake spec to run with *all* adapters

* After discussion with carllerche and ssmoot, they convinced me this
  was the saner approach.  I typically use autospec for routine dev
  and only use rake spec when about to push to the main repo, so this
  won't affect me as much.  I still worry about spec speed and hope
  that some work is done to make the specs run in parallel so that
  we can minimize the length of total time the specs take to run.

engineyard/dm-core,datamapper/dm-core,ar-dm/ardm-core,tillsc/dm-core,alloy/dm-core,tpitale/dm-core