Windows: Change $_SERVER['PATH'] to getenv('path')

See https://www.reddit.com/r/PHP/comments/3sifnj/grumphp_a_php_codequality_composer_plugin/cwy6dma and http://php.net/manual/en/function.getenv.php#99134

Author: MidnightDesign

src/Gitonomy/Git/Repository.php

@@ -118,7 +118,7 @@ public function __construct($dir, $options = array())
             'working_dir' => null,
             'debug' => true,
             'logger' => null,
-            'environment_variables' => $is_windows ? array('PATH' => $_SERVER['PATH']) : array(),
+            'environment_variables' => $is_windows ? array('PATH' => getenv('path')) : array(),
             'command' => 'git',
             'process_timeout' => 3600,
         ), $options);