add not null column restraint to enrollments role_id

Change-Id: I167620165171b6fd0d41e69590e1f3225ac2fe2c
Reviewed-on: https://gerrit.instructure.com/45690
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>