use `expired` module

[lukechilds]

From the readme:

TODO

Support for If-Modified-Since conditional responses (currently only uses ETags)
Better handling of Cache-Control headers
Better handling of Expires headers (currently only uses Cache-Control's max-age)
Pull requests welcome.

Sorry, I'm pretty busy and don't have time to submit a PR, but I wrote a module for checking cache headers you may find useful. It handles both Cache-Control and Expires headers: lukechilds/expired