add webrick in gemfile

Gemfile

@@ -17,6 +17,8 @@ gem "minima", "~> 2.0"
 # uncomment the line below. To upgrade, run `bundle update github-pages`.
 gem "github-pages", group: :jekyll_plugins
 
+gem "webrick"
+
 # If you have any plugins, put them here!
 group :jekyll_plugins do
   gem "jekyll-feed", "~> 0.6"
@@ -31,4 +33,4 @@ gem "wdm", "~> 0.1.0" if Gem.win_platform?
 group :development, :test do
   gem "pry"
   gem "html-proofer"
-end
+end