Also check stable12 and stable13

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

.travis.yml

@@ -49,6 +49,10 @@ matrix:
 
 matrix:
   include:
+    - php: 7
+      env: "DB=sqlite CORE_BRANCH=stable12"
+    - php: 7
+      env: "DB=sqlite CORE_BRANCH=stable13"
     - php: 7
       env: DB=mysql
     - php: 7