Upgrade ws library

[Naktibalda]

Breaking changes in 3.0.0 were:

    Removed the upgradeReq property (#1099).
    Removed unnecessary events (#1100).
    Removed the flags argument from the 'message', 'ping', and 'pong'
    events (#1101).
    The permessage-deflate extension is now disabled by default on the server
    (#1107).

Only flag argument change affects botkit, but flags argument was unused.

[Naktibalda]

@benbrown I made pull request to master branch because lib/Slackbot_worker.js uses websockets.

[benbrown]

thank you!