Revert "Test against the notification-manager branch for now"

This reverts commit 96ea412ae6.

.travis.yml

@@ -8,7 +8,7 @@ php:
 
 env:
   global:
-    - CORE_BRANCH=notification-manager # TODO revert once the manager is merged: - CORE_BRANCH=master
+    - CORE_BRANCH=master
     - APP_NAME=notifications
   matrix:
     - DB=sqlite