title: how can I install v.6.x of socket.io by NPM now?
body: <p>I got lots of problem to work on v.7.x,but now I have to move my
   project to a new server.so,I have to install socket.io again.
   but,when I used "npm install socket.io" I got the last version.My
   project using v.6.x,now I really cant install v.7.x,so,how can I
   install v.6.x by NPM?</p>