Allow failure for stable12/php7.2

Signed-off-by: Julius Härtl <jus@bitgrid.net>

.travis.yml

@@ -9,9 +9,13 @@ php:
     - 7.1
     - 7.2
 env:
-    matrix:
+    - CORE_BRANCH=master DB=mysql
-        - CORE_BRANCH=master DB=mysql
+    - CORE_BRANCH=stable12 DB=mysql 
-        - CORE_BRANCH=stable12 DB=mysql 
+
+matrix:
+    exclude:
+        - php: 7.2
+          env: CORE_BRANCH=stable12 DB=mysql
 
 before_install:
     - wget https://phar.phpunit.de/phpunit-5.7.phar