fix(errors): Remove HTTPUpgradeRequired class

The '426 Upgrade Required' status code is for renegotiating the protocol
used to talk between client and server; the app should rely on the
WSGI server to perform this work.

BREAKING CHANGE: HTTPUpgradeRequired is no longer available, and so
    any apps relying on it will break. This should be a very small
    number of apps, if any, since protocol negotiation should be up
    to the WSGI host, not the app.
Merge pull request #252 from kgriffs/fix-upgrade-required

fix(errors): Remove HTTPUpgradeRequired class