add webrick to gemfile

as recommended by
https://www.earthinversion.com/blogging/how-to-install-jekyll-on-appple-m1-macbook/

Author: SethTisue

Gemfile

@@ -13,3 +13,5 @@ gem 'kramdown-parser-gfm'
 # browser during development. To use, uncomment
 # and do:
 # bundle exec jekyll liveserve --incremental
+
+gem "webrick", "~> 1.7"

Gemfile.lock

@@ -79,6 +79,7 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.7)
     typhoeus (1.4.0)
       ethon (>= 0.9.0)
+    webrick (1.7.0)
     yell (2.2.2)
 
 PLATFORMS
@@ -89,6 +90,7 @@ DEPENDENCIES
   jekyll-redirect-from
   jekyll-scalafiddle
   kramdown-parser-gfm
+  webrick (~> 1.7)
 
 BUNDLED WITH
    2.3.6