Changelog_SSCformat.txt said we were on v0.82, but Song.h said we were on v0.81... fix that.

also update FILE_CACHE_VERSION to regenerate cache for #PREVIEWVID tag (if it isn't already)

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 = 216;
+const int FILE_CACHE_VERSION = 217;
 
 /** @brief How long does a song sample last by default? */
 const float DEFAULT_MUSIC_SAMPLE_LENGTH = 12.f;