Fix if 'git' instead of 'git.exe' as command is used on windows (#168)

RoboFile.php

@@ -65,10 +65,18 @@ class RoboFile extends \Robo\Tasks
 	private function getExecutableExtension()
 	{
 		if ($this->isWindows())
+		{
+			// check wehter git.exe or git as command should be used,
+			// as on window both is possible
+			if(!$this->_exec('git.exe --version')->getMessage())
+			{
+				return '';
+			}
+			else
 			{
 				return '.exe';
 			}
-
+		}
 		return '';
 	}