Fixed bundle exec jekyll error

Gemfile

@@ -1,11 +1,13 @@
 source 'https://rubygems.org'
 
 group :development, :test do
-  gem 'jekyll', '~> 3.8.5'
+  gem 'jekyll'
   gem 'jekyll-redirect-from', '~> 0.14.0'
   gem 'jekyll-sitemap', '~> 1.2.0'
   gem 'jekyll-toc', '~> 0.9.1'
   gem 'wdm', '~> 0.1.1', :install_if => Gem.win_platform?
+  gem "webrick"
+  gem "kramdown-parser-gfm"
 end
 
 group :jekyll_plugins do