Refactor code to work with prefixed Sabberworm\CSS\Parser

[MAILPOET-1761]

lib/Config/RequirementsChecker.php

@@ -34,7 +34,6 @@ class RequirementsChecker {
     '\Cron\CronExpression',
     '\Html2Text\Html2Text',
     '\csstidy',
-    '\Sabberworm\CSS\Parser'
   );
 
   function __construct($display_error_notice = true) {

lib/Form/Util/Styles.php

@@ -1,7 +1,7 @@
 <?php
 namespace MailPoet\Form\Util;
 
-use Sabberworm\CSS\Parser as CSSParser;
+use MailPoetVendor\Sabberworm\CSS\Parser as CSSParser;
 
 class Styles {
   public $styles;