spec: firefox 26 upgrade selenium

Change-Id: I5207da68a1ae77c3255ddaa9e15b07632d930520
Reviewed-on: https://gerrit.instructure.com/29669
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Hannah Bottalla <hannah@instructure.com>
Product-Review: Hannah Bottalla <hannah@instructure.com>
QA-Review: Hannah Bottalla <hannah@instructure.com>
Tested-by: Hannah Bottalla <hannah@instructure.com>
Reviewed-by: Bryan Madsen <bryan@instructure.com>