Use FormatJson in font repo generator script

The benefit of this is that '/' is no longer escaped, making
ext.uls.webfonts.repository.js file 376 bytes smaller. I expect
that the difference in compressed size is neglible.

Change-Id: I77c0b88fa23f2245f09615c354546b6a9ebf650c

scripts/compile-font-repo.php

@@ -31,7 +31,7 @@ class CompileFontRepo extends Maintenance {
 
 		$list = $compiler->getRepository();
 
-		$json = json_encode( $list );
+		$json = FormatJson::encode( $list );
 		$js = <<<JAVASCRIPT
 // Do not edit! This file is generated from data/fontrepo by data/fontrepo/scripts/compile.php
 ( function ( $ ) {