Setting max version for warp, as API changed in 2.1

Just a small fix to make sure haskell-hubbub builds in a fresh sandbox. Changes to libraries:

* In Web.Scotty, reqHeader is renamed header
* In Network.Wai.Handler.WebSockets, intercept is gone (or renamed & types have all changed)

It would probably be better to update haskell-hubbub, but that's a bit beyond me ATM.