Regeneration fails without network access

### Before submitting an issue, please be sure to

- [X] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [X] Update to the latest Gem version (either `gem update github-pages` or `bundle update github-pages`)

### This issue affects

- [ ] The site generated by GitHub Pages
- [X] Building sites locally

### What did you do (e.g., steps to reproduce)

`bundle exec jekyll serve` without network connectivity.

### What did you expect to happen?

The page to regenerate when local changes are made on disk.

### What happened instead?

The page initially generates and serves fine, but on regeneration I get:
```
% bundle exec jekyll serve
Configuration file: /home/user/qubesos.github.io/_config.yml
Configuration file: /home/user/qubesos.github.io/_config.yml
Source: /home/user/qubesos.github.io
Destination: /home/user/qubesos.github.io/_site
Incremental build: disabled. Enable with --incremental
Generating...
done in 9.53 seconds.
Auto-regeneration: enabled for '/home/user/qubesos.github.io'
Configuration file: /home/user/qubesos.github.io/_config.yml
Server address: http://127.0.0.1:4000/
Server running... press ctrl-c to stop.
Regenerating: 1 file(s) changed at 2016-12-11 08:05:11 GitHub Metadata: No GitHub API authentication could be found. Some fields may be missing or have incorrect data.
GitHub Metadata: Failed to open TCP connection to api.github.com:443 (No route to host - connect(2) for "api.github.com" port 443)
GitHub Metadata: Failed to open TCP connection to api.github.com:443 (No route to host - connect(2) for "api.github.com" port 443)
GitHub Metadata: Failed to open TCP connection to api.github.com:443 (No route to host - connect(2) for "api.github.com" port 443)
GitHub Metadata: Failed to open TCP connection to api.github.com:443 (No route to host - connect(2) for "api.github.com" port 443)
GitHub Metadata: Failed to open TCP connection to api.github.com:443 (No route to host - connect(2) for "api.github.com" port 443)
...
```
indefinitely

### Additional information

I am trying to do build and serve the page locally without requiring network access.

The above error comes from the `jekyll-github-metadata` plugin, but the site does not use any `{{ site.github.* }}` whatsoever, so it would be nice to be able to simply disable that plugin somehow if not required or desired.

* Link to the live site (if applicable):
- https://www.qubes-os.org/

* Link to the source repo (if applicable):
- https://github.com/QubesOS/qubesos.github.io