only enable non-blocking postgres behavior on rails 6
only enable non-blocking postgres behavior on rails 6

it seems to be causing hanging issues on rails 5.2

Change-Id: I9e53d17670579113c0adfa318dd2b7365de9c0d9
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/258364
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Reviewed-by: Jeffrey Johnson <7a0f98bfe168bc29d5611ba0275e88567b492abc@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms