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

appinfo/info.xml

@@ -36,7 +36,7 @@
 	<repository>https://github.com/nextcloud/activity.git</repository>
 
 	<dependencies>
-		<nextcloud min-version="18" max-version="18"/>
+		<nextcloud min-version="19" max-version="19"/>
 	</dependencies>
 
 	<background-jobs>