spec: rails3 models/plugin_settings_spec

Change-Id: I7d9e75c46fc49ab2b86020e7f9cdb8f2c57ee24f
Reviewed-on: https://gerrit.instructure.com/29637
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>
QA-Review: Jon Willesen <jonw@instructure.com>