Express 4 support

[kmalakoff]

I have been trying to update our applications to Express 4.

Assert rack doesn't seem to be working. I started to try to get it working (https://github.com/kmalakoff/asset-rack), but realized that I'm out of my depth. I just don't know enough about writing express middleware.

Maybe this repo could be a start of an upgrade? Or is asset-rack not planning to be upgraded? If not, what do you recommend as an alternative?