bump pg to v0.15.1 to silence warnings

changelog available here: http://git.io/wyHj1A

Change-Id: Ie904c2dbd8bc38934af60ed30760714ed84d2d31
Reviewed-on: https://gerrit.instructure.com/39258
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Paul Hinze <paulh@instructure.com>
QA-Review: Paul Hinze <paulh@instructure.com>