bump switchman to 1.2.20

fixes CNVS-14894

this fixes a problem finding shadow objects through has_through associations,
and also makes a tweak for matching global ids to try to better support 32 bit
architectures.

test plan:
- set up a course on a sharded account
- create one student in the site admin account and one in the new shard account
- enroll them both in the course
- create a Peer Review Assignment in the courses
- assign them to each peer review the other
- it should work

Change-Id: I8614791d145d11ef8ab5666234e2e7443f71f065
Reviewed-on: https://gerrit.instructure.com/42975
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>