Bump cache version for XML changes

The XML patchset includes more items in the FGCHANGES line, which is
stored in cache.  Previous caches wouldn't include these entries.

Sorry 'bout that folks.

src/Song.cpp

@@ -42,7 +42,7 @@
  * @brief The internal version of the cache for StepMania.
  *
  * Increment this value to invalidate the current cache. */
-const int FILE_CACHE_VERSION = 215;
+const int FILE_CACHE_VERSION = 216;
 
 /** @brief How long does a song sample last by default? */
 const float DEFAULT_MUSIC_SAMPLE_LENGTH = 12.f;