It is a follow up to issue
#18, the elpa package provide a new release of "org-schedule", but an old one for org-drill (2.3.7) which is incompatible with org-schedule. It is stiil true in the version of org-plus-contrib 20141201. I had some difficulty to understand why as the org git repository correctly include an up-to-date version of org-drill. But I have checked the maint branch of git repo, and this one has still the old org-drill. I guess that elpa is packaged with maint and not master, so it would be needed to merge maint with a recent version of org-drill. Of course it is to be asked to the maintainers of org git; but I ignore how it can be asked, and what is the policy of maint regarding the contrib.
May be Paul you can obtain it, It is truly a bad thing or ordinary people (I mean not emacs developers) that rely on packages to get their emacs libraries.