Update client-side socket.io CDN link to current version.

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:8-alpine
+FROM node:10-alpine
 
 EXPOSE 3000
 

views/index.html

@@ -19,7 +19,7 @@
 <script src="addon/search/searchcursor.js"></script>
 <script src="addon/edit/matchbrackets.js"></script>
 <script src="keymap/vim.js"></script>
-<script src="https://cdn.socket.io/socket.io-1.4.5.js"></script>
+<script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/2.1.1/socket.io.slim.js"></script>
 <script src="//cdn.jsdelivr.net/pouchdb/6.0.7/pouchdb.min.js"></script>
 <style type="text/css">
     html, body { height: 100%; margin: 0; padding: 0; }