fix for the empty $name issue (#94)

fix for the empty $name issue, e.g.    
The [] symbolic link has been removed.

Author: vitr

cli/valet.php

@@ -108,7 +108,7 @@
  * Unlink a link from the Valet links directory.
  */
 $app->command('unlink [name]', function ($name) {
-    Site::unlink($name ?: basename(getcwd()));
+    Site::unlink($name = $name ?: basename(getcwd()));
 
     info('The ['.$name.'] symbolic link has been removed.');
 })->descriptions('Remove the specified Valet link');