fix call to microtime (#1)

the previous incantation would cast the `microtime()` output to `int` before multiplying, which only allowed for 1-second resolution (the last three digits of `$now` were always zero)

casting to `int` after multiplying allows the intended millisecond resolution

src/Ulid.php

@@ -53,7 +53,7 @@ private function __construct($time, $randomness)
      */
     public static function generate()
     {
-        $now = (int) microtime(true) * 1000;
+        $now = intval(microtime(true) * 1000);
         $duplicateTime = $now === static::$lastGenTime;
 
         $timeChars = '';