Switch utf8_encode to mb_convert_encoding

`utf8_encode()` is not included by default with PHP 7 now. Since
Laravel requires the MB extension, it’s safe to switch over to
`mb_convert_encoding()`

composer.json

@@ -15,6 +15,7 @@
     },
     "require": {
         "php": ">=5.4.0",
+        "ext-mbstring": "*",
         "illuminate/http": "^5.0",
         "piwik/referrer-spam-blacklist": "@dev"
     },

src/Middleware/BlockReferralSpam.php

@@ -15,15 +15,15 @@ class BlockReferralSpam
      */
     public function handle($request, Closure $next)
     {
-        $referer = utf8_encode($request->headers->get('referer'));
+        $referer = mb_convert_encoding($request->headers->get('referer'), 'UTF-8');
         $spammerList = config('app.referral_spam_list_location', base_path('vendor/piwik/referrer-spam-blacklist/spammers.txt'));
 
         // Make sure there's a referrer
         if (!empty($referer) && file_exists($spammerList)) {
             $blockedHosts = file($spammerList, FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
 
             foreach ($blockedHosts as $i => $host) {
-                $blockedHosts[$i] = trim(utf8_encode($host));
+                $blockedHosts[$i] = trim(mb_convert_encoding($host, 'UTF-8'));
             }
 
             preg_match('/^(?:https?:\/\/)?(?:[^@\n]+@)?(?:www\.)?([^:\/\n]+)/', $referer, $matches);