Update socket.io to 1.7.3
Update socket.io to 1.7.3

That in turn upgrades engine.io to 1.8.2. This fixes a crash for me when
running behind a traefik reverse proxy.
https://github.com/socketio/engine.io/issues/465

lpagliari/etherpad-lite,lpagliari/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,joequant/etherpad-lite,lpagliari/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,aptivate/etherpad-lite,storytouch/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,joequant/etherpad-lite,joequant/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,aptivate/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,lpagliari/etherpad-lite,ether/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,aptivate/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,aptivate/etherpad-lite