Update stable18 target versions

Signed-off-by: Joas Schilling <coding@schilljs.com>

Author: nickvergessen

.travis.yml

@@ -9,7 +9,7 @@ services:
 
 env:
   global:
-    - CORE_BRANCH=master
+    - CORE_BRANCH=stable18
     - APP_NAME=activity
   matrix:
     - DB=sqlite