Fix easily security alerts for Minimal Mistakes theme

with git command

• prerequisites

git remote add upstream git@github.com:mmistakes/minimal-mistakes.git  # in your github repository
git fetch --all

• update files from mmistakes v.4.19.1 according to Github network alerts

git checkout 4.19.1 -- minimal-mistakes-jekyll.gemspec package.json package-lock.json
git commit -m "[update] from minimal mistakes 4.19.1 release"