add sis_course_id to sections api attributes

fixes SIS-277

Change-Id: I06bfef5c18427b8db311ab433d5370fdb8c14bc5
Reviewed-on: https://gerrit.instructure.com/34955
Reviewed-by: Duane Johnson <duane@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Eric Adams <eadams@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>