Don't use PHP's fmod

PHP's fmod has some quirks.  The private fmod function wraps it to iron out some of those quirks but ultimately it's better to just not use it.

src/JsonSchema/Constraints/NumberConstraint.php

@@ -71,7 +71,7 @@ public function check(&$element, $schema = null, JsonPointer $path = null, $i =
     private function fmod($number1, $number2)
     {
         $number1 = abs($number1);
-        $modulus = fmod($number1, $number2);
+        $modulus = ($number1 - round($number1 / $number2) * $number2);
         $precision = abs(0.0000000001);
         $diff = (float) ($modulus - $number2);