remove webrat, and fix pseudonym sessions controller spec for rails 3

webrat is abandonware, and doesn't work properly with rails 3

Change-Id: I34db6c150b6715e33a30d531941b76458090ce63
Reviewed-on: https://gerrit.instructure.com/31457
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>