Updated docs for Express: no need in NODE_ENV

raven-node documentation (https://github.com/mattrobenolt/raven-node) now says:

> We don't infer this from NODE_ENV automatically anymore. It's up to you to implement whatever logic you'd like.

Therefore sentry docs do not need to mention NODE_ENV configuration anymore.

Also `app.error` doesn't seem to work now, it should be `app.use`.
Merge pull request #1103 from arty-name/patch-3

Updated docs for Express: no need in NODE_ENV