Added whitelist to remove warning for upcoming phpunit 4.8

phpunit.xml

@@ -14,6 +14,11 @@
             <directory>./tests/</directory>
         </testsuite>
     </testsuites>
+    <filter>
+        <whitelist>
+            <directory suffix=".php">app/</directory>
+        </whitelist>
+    </filter>
     <php>
         <env name="APP_ENV" value="testing"/>
         <env name="CACHE_DRIVER" value="array"/>