fix simply_versioned for rails 4

 * attr_accessible defaults to none
 * method_missing will get called for yaml= before
   versionable_type is even populated; don't choke on it

Change-Id: I1d6fcf103452138f73c09bccada1dafa18f02fd4
Reviewed-on: https://gerrit.instructure.com/42070
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>