Gem needs an update

I can't get this to work. It complains about Jekyll being the wrong version.

```
Fetching gem metadata from https://rubygems.org/............
Resolving dependencies...
Bundler could not find compatible versions for gem "jekyll":
  In Gemfile:
    jekyll (~> 4.2.2)

    jekyll-shell-theme was resolved to 0.1.0, which depends on
      jekyll (~> 3.8.5)
```