Upgrade data_objects dependency
Fix db:create and db:drop rake tasks for mysql
Update dm-rails to work with rails 4
Remove ruby 1.8 from the supported ruby versions
Add ruby-head to the allowed failures matrix

* There is something messed up on travis, so this is a temporary
  work-around.
Remove 1.9.2 and ree from list of rubies to test against
Update travis notifications to only warn on failure
Merge remote-tracking branch 'datamapper/release-1.2' into release-1.2