Enforce memory limit in phpunit.xml (#320)

This will help highlight memory regressions, make it easier for newcomers to get started with the codebase w/o editing php.ini defaults (128M), and also keep things consistent between local and travis runs.

Author: mousetraps

phpunit.xml.dist

@@ -11,4 +11,7 @@
             <directory>./src</directory>
         </whitelist>
     </filter>
+    <php>
+        <ini name="memory_limit" value="256M"/>
+    </php>
 </phpunit>