update gemfile, add webrick to dependencies (#49)

* update Gemfile.lock

* add webrick to dependencies to fix potential problem when using Ruby version 3.0.0 or higher
https://github.com/jekyll/jekyll/blob/dbbfc5d48c81cf424f29c7b0eebf10886bc99904/docs/_docs/index.md

Gemfile

@@ -8,4 +8,5 @@ group :jekyll_plugins do
   gem "jekyll-redirect-from"
   gem "jekyll-feed"
   gem "jekyll-paginate-v2"
-end
+end
+gem "webrick", "~> 1.8"

Gemfile.lock

@@ -1,18 +1,18 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    addressable (2.8.5)
+    addressable (2.8.6)
       public_suffix (>= 2.0.2, < 6.0)
     colorator (1.1.0)
-    concurrent-ruby (1.2.2)
+    concurrent-ruby (1.2.3)
     em-websocket (0.5.3)
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0)
     eventmachine (1.2.7)
-    ffi (1.15.5)
+    ffi (1.16.3)
     forwardable-extended (2.6.0)
     http_parser.rb (0.8.0)
-    i18n (1.14.1)
+    i18n (1.14.4)
       concurrent-ruby (~> 1.0)
     jekyll (4.2.2)
       addressable (~> 2.4)
@@ -46,13 +46,13 @@ GEM
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
     liquid (4.0.4)
-    listen (3.8.0)
+    listen (3.9.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.4.0)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
-    public_suffix (5.0.3)
+    public_suffix (5.0.4)
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
@@ -64,6 +64,7 @@ GEM
     terminal-table (2.0.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     unicode-display_width (1.8.0)
+    webrick (1.8.1)
 
 PLATFORMS
   ruby
@@ -74,6 +75,7 @@ DEPENDENCIES
   jekyll-paginate-v2
   jekyll-redirect-from
   jekyll-sitemap
+  webrick (~> 1.8)
 
 BUNDLED WITH
    2.4.13