Fix 'undefined index' warning.

I got warnings running PHP 5.4 built-in server as a test server for my API:

PHP Notice:  Undefined index: REQUEST_METHOD in /home/gytis/Projects/pomodor-api/src/vendor/slim/slim/Slim/Environment.php on line 125
PHP Notice:  Undefined index: REMOTE_ADDR in /home/gytis/Projects/pomodor-api/src/vendor/slim/slim/Slim/Environment.php on line 129
PHP Notice:  Undefined index: REQUEST_URI in /home/gytis/Projects/pomodor-api/src/vendor/slim/slim/Slim/Environment.php on line 146
PHP Notice:  Undefined index: REQUEST_URI in /home/gytis/Projects/pomodor-api/src/vendor/slim/slim/Slim/Environment.php on line 151
PHP Notice:  Undefined index: SERVER_NAME in /home/gytis/Projects/pomodor-api/src/vendor/slim/slim/Slim/Environment.php on line 162
PHP Notice:  Undefined index: SERVER_PORT in /home/gytis/Projects/pomodor-api/src/vendor/slim/slim/Slim/Environment.php on line 165