Fix package.json bitrot.

Most importantly, node-websocket-server has been unpublished from npm
and now fails to install. BrowserQuest really ought to be ported to the
'ws' package which supersedes websocket and websocket-server, but for
now the server code can continue to function by installing
websocket-server directly from GitHub.

I also encountered issues with version numbers for 'sanitizer' and
'memcache', which depending on the version 'latest' seems to solve.