Update cssmin requirement

Author: Christoph Singer

PrettyMin.php

@@ -2,6 +2,7 @@
 namespace Wa72\HtmlPrettymin;
 
 use JSMin\JSMin;
+use tubalmartin\CssMin\Minifier as CSSmin;
 use Symfony\Component\OptionsResolver\OptionsResolver;
 
 /**
@@ -183,7 +184,7 @@ protected function minifyCss()
             $code = $element->nodeValue;
             $element->nodeValue = '';
             if (trim($code)) {
-                $min = new \CSSmin();
+                $min = new CSSmin();
                 if (trim($code)) {
                     $code = trim($min->run($code));
                 }

composer.json

@@ -4,8 +4,9 @@
     "type":"library",
     "license":"MIT",
     "require": {
+        "php": ">=5.4",
         "mrclay/jsmin-php": "^2.3",
-        "tubalmartin/cssmin": "^2.4",
+        "tubalmartin/cssmin": "^4",
         "symfony/options-resolver": ">=2.3"
     },
     "require-dev": {