make dev suffix rank work for uppercase too

Hi, I wanted to release a new version after BETA, with name RC, but they were the same.

I digged deeper and discoverd the score is incorrectly interpreted for uppercase.

- rc -> 3
- RC -> 0
- beta -> 2
- BETA -> 0

src/PreReleaseSuffix.php

@@ -60,6 +60,8 @@ public function isGreaterThan(PreReleaseSuffix $suffix): bool {
      * @param $value
      */
     private function mapValueToScore($value): int {
+        $value = strtolower($value);
+
         if (\array_key_exists($value, self::valueScoreMap)) {
             return self::valueScoreMap[$value];
         }