Improve built-in Jekyll build commands

#359 identified a couple of issues for building certain Jekyll apps.

* By default, Jekyll builds with `JEKYLL_ENV=development`. Apps that depend on this variable to check for `production` will not work as expected
* It should run `bundle exec jekyll build` instead of `jekyll build` if there's a Gemfile

Perhaps the build command should be: `JEKYLL_ENV=production bundle exec jekyll build`

There's also a question of whether we should be using Ruby 2.x or 3.x.

@qianz2 @arroyc