Update app.js and mitigate js-yaml issue

Changes

• Missing require('fs') in server/app.js See issue #135
• Patched read-config to mitigate vulnerability in js-yaml
  • issue not exploitable on webssh2 implementation
  • patched anyway
  • sending my patch upstream to read-config, webssh2 package.json points to patched version in my repository https://github.com/billchurch/nodejs-read-config
  • See nodeca/js-yaml#475 for more detail