cirrus fails when no diffs

[nick-gray]

When no changes are detected, exit code 1 comes out. Either make this 0 or some specific code we can ignore.

[Thu Apr 20 2017 15:07:09 GMT+1000 (AEST) :cloudformation: :barely_sunny:] There was an error creating your stack diff.
[Thu Apr 20 2017 15:07:09 GMT+1000 (AEST) :cloudformation: :barely_sunny:] Have a look at the AWS console for more details
[Thu Apr 20 2017 15:07:09 GMT+1000 (AEST) :cloudformation: :barely_sunny:] ChangeSet Name: dat_test_changeset
[Thu Apr 20 2017 15:07:09 GMT+1000 (AEST) :cloudformation: :barely_sunny:] Error: The submitted information didn't contain changes. Submit different information to create a change set.
[Thu Apr 20 2017 15:07:09 GMT+1000 (AEST) :cloudformation: :barely_sunny:] Exited with code 1