Change default cache location back to original

In this commit 80164cb the `assets` folder looks to be accidentally removed from the cache file store. This causes problems if you're specifically moving that folder between builds.

Author: schneems

lib/sprockets/railtie.rb

@@ -98,7 +98,7 @@ def configure(&block)
 
     config.assets.configure do |env|
       env.cache = Sprockets::Cache::FileStore.new(
-        "#{env.root}/tmp/cache",
+        "#{env.root}/tmp/cache/assets",
         config.assets.cache_limit,
         env.logger
       )