Fixed default phpunit.xml.dist: add filter/whitelist

If we don't do that, phpunit is not able to compute the code coverage.
We get the following error without it:

```
Error:         Incorrect whitelist config, no code coverage will be generated.
```

phpunit/phpunit/4.7/phpunit.xml.dist

@@ -22,4 +22,10 @@
             <directory>tests/</directory>
         </testsuite>
     </testsuites>
+
+    <filter>
+        <whitelist>
+            <directory>./src/</directory>
+        </whitelist>
+    </filter>
 </phpunit>