fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:ws:20171108

Latest report for billchurch/webssh2:
https://snyk.io/test/github/billchurch/webssh2

package.json

@@ -34,7 +34,7 @@
     "express-session": "^1.15.6",
     "morgan": "^1.9.0",
     "read-config": "^1.6.0",
-    "socket.io": "^1.7.4",
+    "socket.io": "^2.0.0",
     "ssh2": "^0.5.5",
     "validator": "^9.0.0"
   },