Loosen commonmarker dependency

[patcon]

Hiya! Thanks for all the work on this!

Could you loosen this dep so that tasklist extensions will work? Thanks!
https://github.com/github/jekyll-commonmark-ghpages/blob/master/jekyll-commonmark-ghpages.gemspec#L18

[hugmanrique]

This repo seems inactive, so I've forked it and loosened the dep on https://github.com/hugmanrique/jekyll-commonmark-ghpages. Install it using

group :jekyll_plugins do
  gem "jekyll-commonmark-ghpages", :git => 'git@github.com:hugmanrique/jekyll-commonmark-ghpages.git'
end

Note: PRs should still be directed to this repo, I don't plan on actively maintaining mine.