fix group category role_category_for_context saving for rails 3

to save without validation, in rails 3,
you have to pass :validate => false instead of simply false

Change-Id: I28bae42594495dafb94b36d01e075a13f5a5f9a8
Reviewed-on: https://gerrit.instructure.com/28476
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>