Disable sign-on-o-tron tests on staging

Sign-on-o-tron wont work in staging due to the way dns is setup. This
change brings things into line with the other sign-on-o-tron test which
is also skipped on staging.
Merge pull request #28 from alphagov/disable-admin-app-tests-on-staging

Disable sign-on-o-tron tests on staging