spec: simple_cov db dir exclusion

Change-Id: Ia71e243beca09c899bf34cb8ae1b2fe33aec1708
Reviewed-on: https://gerrit.instructure.com/36330
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Shawn Meredith <shawn@instructure.com>
Tested-by: Shawn Meredith <shawn@instructure.com>