Add webrick to Gemfile, add github metadata to config

[iBug]

Fixes mmistakes/minimal-mistakes#3774

[JJTech0130]

What's github: [metadata] do? I get that you need to add webrick so that you can serve it locally, but I don't get the metadata thing.

[jasonhemann]

I believe @iBug's other patch is to resolve the following warning one receives when serving the site locally.

GitHub Metadata: No GitHub API authentication could be found. Some fields may be missing or have incorrect data.