Website: bundle update

This issue relates to the website (gh-pages branch).

I've been confused with `jekyll serve` not reporting an URL to visit the dev website.

It takes time to learn the default port of Jekyll and more time to find out that the site should be visited at a sub-page.

The reason why the URL is not being displayed [has turned out to be](https://github.com/jekyll/jekyll/issues/2461) that we're using a year-old version of Jekyll.

Thus, `bundle update`, yay!

Before:

```
$ bundle exec jekyll serve
Configuration file: /media/hdd/Dropbox/Code/Sandbox/ember-cli3/_config.yml
            Source: /media/hdd/Dropbox/Code/Sandbox/ember-cli3
       Destination: /media/hdd/Dropbox/Code/Sandbox/ember-cli3/_site
      Generating... done.
  Server running... press ctrl-c to stop.
```

After:

```
$ bundle exec jekyll serve
Configuration file: /media/hdd/Dropbox/Code/Sandbox/ember-cli3/_config.yml
            Source: /media/hdd/Dropbox/Code/Sandbox/ember-cli3
       Destination: /media/hdd/Dropbox/Code/Sandbox/ember-cli3/_site
      Generating... 
                    done.
Configuration file: /media/hdd/Dropbox/Code/Sandbox/ember-cli3/_config.yml
    Server address: http://0.0.0.0:4000/ember-cli/
  Server running... press ctrl-c to stop.
```

People doing `gem install jekyll` and `jekyll serve` will not notice any difference, they most probably have already got a recent version of Jekyll.

People doing `bundle install` and `bundle exec jekyll serve' will be asked to `bundle update`.