Minimal support for Ruby 3

This isn't officially supported by Jekyll and is a bit hacky, so
I'm deliberately not documenting the support, however as recent
Ubuntu versions now ship with Ruby 3 (not 2.7) and Jekyll isn't
showing any signs of moving towards supporting Ruby 3 we're not
left with many alternatives.

This works around:
- github/pages-gem#752
- envygeeks/pathutil#5

Gemfile

@@ -19,3 +19,6 @@ gem 'html-proofer'
 
 # Avoid polling on windows
 gem 'wdm', '>= 0.1.0'
+
+# For local Ruby 3 support; works around https://github.com/github/pages-gem/issues/752
+gem "webrick", "~> 1.7"

Gemfile.lock

@@ -98,6 +98,7 @@ GEM
     tzinfo-data (1.2018.5)
       tzinfo (>= 1.0.0)
     wdm (0.1.1)
+    webrick (1.7.0)
     yell (2.2.2)
 
 PLATFORMS
@@ -118,6 +119,7 @@ DEPENDENCIES
   tzinfo
   tzinfo-data
   wdm (>= 0.1.0)
+  webrick (~> 1.7)
 
 BUNDLED WITH
    2.3.6

Rakefile

@@ -11,6 +11,9 @@ task :dependencies do
     sh('bundle config set --local path "gems"')
   end
   sh('bundle install')
+
+  # Fix pathutil on Ruby 3; works around https://github.com/envygeeks/pathutil/pull/5
+  sh('sed -i.bak "s/, kwd/, **kwd/" $(bundle exec gem which pathutil)')
 end
 
 task :spelling_dependencies do