Set the default timezone to avoid errors in hhvm

Added a default timezone to the bootstrap.php to fix errors in HHVM
tests, which was introduced when Travis removed the default `/etc/hhvm/php.ini`
in https://github.com/travis-ci/travis-ci/issues/2523#issuecomment-59303763

tests/bootstrap.php

@@ -1,6 +1,7 @@
 <?php
 
 error_reporting(E_ALL | E_STRICT);
+date_default_timezone_set('UTC');
 
 // Ensure that composer has installed all dependencies
 if (!file_exists(dirname(__DIR__) . '/vendor/autoload.php')) {