Turn off cache for assets pipeline

To replace an asset file with older modification timestamp,
the precompile step might skip it, thinking the compiled version
(cached in /tmp/cache/assets with newer timestamp) is up-to-date.

`rake assets:clean` does not solve the problem because it only
removes old compiled assets but not the cache. This may be an issue
in rails/sprockets-rails.
See https://github.com/rails/sprockets-rails/issues

Disable the assets pipeline cache to hack the issue.