spec: fix some travis spec errors

Attempt to fix our sis batch tempfile usage, which wasn't compatible
with the travis environment.

Found more SAML specs to mark as pending when SAML isn't enabled.

Use webmock instead of our own home-grown solution, since Travis
enforces webmock.

Select only the users we're interested in sorting, to remove dependency
on empty db.

refs CNVS-10467

Change-Id: Icf9b093c6cc4a6b7b19da9dd2abab9f39457f817
Reviewed-on: https://gerrit.instructure.com/28949
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>