Revert "Rename any "- number" files (fix for StaticCache)"

This reverts commit 3532b8492af6e3dd1d8956bce15c796977127bad.
Use Last-Modified & asset_hash (not cache_buster)

 * Use my patch for Rack::StaticCache which a) fixes files ending with a
   dash followed by a number and b) doesn't unnecessarily delete
   Last-Modified headers (pending PR open)
 * Use middleman's asset_hash extension rather than cache_buster;
   cache_buster appended a query string which isn't always honored by
   proxies
 * Re-enable Rack::StaticCache