fix: Gemfile to reduce vulnerabilities
fix: Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-KRAMDOWN-1087436
proinsias/proinsias.github.io,proinsias/proinsias.github.io,proinsias/proinsias.github.io,proinsias/proinsias.github.io