Enforce difficulty meter ranges.

WARNING! This invalidates the cache!
TODO: Make MAX_METER a common metric?

src/Song.cpp

@@ -41,7 +41,7 @@
  * @brief The internal version of the cache for StepMania.
  *
  * Increment this value to invalidate the current cache. */
-const int FILE_CACHE_VERSION = 167;
+const int FILE_CACHE_VERSION = 168;
 
 /** @brief How long does a song sample last by default? */
 const float DEFAULT_MUSIC_SAMPLE_LENGTH = 12.f;