Fix mb_substr third param to support php 5.3

src/Faker/Provider/zh_TW/Text.php

@@ -129,7 +129,9 @@ protected static function appendEnd($text)
 
         // extract the last char of $text
         if ($mbAvailable) {
-            $last = mb_substr($text, mb_strlen($text, static::$encoding) - 1, null, static::$encoding);
+            // in order to support php 5.3, third param use 1 instead of null
+            // https://secure.php.net/manual/en/function.mb-substr.php#refsect1-function.mb-substr-changelog
+            $last = mb_substr($text, mb_strlen($text, static::$encoding) - 1, 1, static::$encoding);
         } else {
             $chars = static::utf8Encoding($text);
             $last = $chars[count($chars) - 1];