Don't hardcode the server's hostname

[MarkRedeman]

This fixes a usability bug where the server is run inside of a Docker container (and probably also when inside of VM?). Since the container is run on a different IP the server wouldn't listen to any incoming requests.

This does come with the downside that the server will now accept connections on any IP address.

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In case anyone is interested, this is the docker-compose.yml file that I'm using to install and serve my slides:

version: '2'

services:
  npm:
    image: node:6
    volumes:
      - .:/usr/src/app
    working_dir: /usr/src/app
    ports:
      - "3000:3000"
    command: [npm, run, start]