Fixed channels/webhooks tests

no issue

- i saw that the channels routing tests were sometimes not running in Travis

e.g. https://travis-ci.org/TryGhost/Ghost/jobs/311303939
> Channel Routes
> Unhandled rejection Error: select "webhooks".* from "webhooks" where "event" = 'subscriber.added' - SQLITE_ERROR: no such table: webhooks

core/test/functional/routes/apps/subscribers/routing_spec.js

@@ -24,6 +24,8 @@ describe('Subscriber: Routing', function () {
         });
     });
 
+    after(testUtils.teardown);
+
     after(function () {
         return ghostServer.stop();
     });