Allow running local phpunit

tests/bootstrap.php

@@ -6,5 +6,8 @@
  * Bootstraper for PHPUnit tests.
  */
 error_reporting(E_ALL | E_STRICT);
-$loader = require_once __DIR__ . '/../vendor/autoload.php';
+// Can be required more than once to allow running
+// phpunit installed with Composer
+// http://stackoverflow.com/a/12798022
+$loader = require __DIR__ . '/../vendor/autoload.php';
 $loader->add('Whoops\\', __DIR__);