Update codebase to patch kramdown vulnerability.

[jestill]

I have a dependabot (https://dependabot.com/) scan running on repos I am attached to. It identified a high severity vulnerability in this code base. I pushed a button to fix it. I think that this should come through as a pull request.

[jestill]

Info from dependabot is

CVE-2020-14001
high severity
Vulnerable versions: < 2.3.0
Patched version: 2.3.0
The kramdown gem before 2.3.0 for Ruby processes the template option inside Kramdown documents by default, which allows unintended read access (such as template="/etc/passwd") or unintended embedded Ruby code execution (such as a string that begins with template="string://<%= `). NOTE: kramdown is used in Jekyll, GitLab Pages, GitHub Pages, and Thredded Forum.