Support jekyll 4.0.0

[hanju-jo]

Jekyll 4.0.0 has released.

• Resolve dependency issue such as 👇👇👇
  Dependency Error: Yikes! It looks like you don't have jekyll-feed or one of its dependencies installed. In order to use Jekyll as currently configured, you'll need to install this gem. If you've run Jekyll with bundle exec, ensure that you have included the jekyll-feed gem in your Gemfile as well. The full error message from Ruby is: 'cannot load such file -- jekyll-feed' If you run into trouble, you can find helpful resources at https://jekyllrb.com/help/!
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  Jekyll 4.0.0 Please append --trace to the serve command
  for any additional information or backtrace.
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