Bump html-proofer from 3.18.5 to 3.18.6

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.18.5 to 3.18.6.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.18.5...v3.18.6)

Gemfile

@@ -11,6 +11,6 @@ gem "jekyll-sitemap", "~> 1.4"
 gem "rake", "~> 13.0"
 
 group :test do
-    gem 'html-proofer', '~> 3.18.5'
+    gem 'html-proofer', '~> 3.18.6'
     gem 'yaml-lint', '~>0.0.7'
 end

Gemfile.lock

@@ -13,7 +13,7 @@ GEM
     eventmachine (1.2.7)
     ffi (1.14.2)
     forwardable-extended (2.6.0)
-    html-proofer (3.18.5)
+    html-proofer (3.18.6)
       addressable (~> 2.3)
       mercenary (~> 0.3)
       nokogumbo (~> 2.0)
@@ -59,15 +59,17 @@ GEM
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.4.0)
-    mini_portile2 (2.4.0)
-    nokogiri (1.10.10)
-      mini_portile2 (~> 2.4.0)
+    mini_portile2 (2.5.0)
+    nokogiri (1.11.1)
+      mini_portile2 (~> 2.5.0)
+      racc (~> 1.4)
     nokogumbo (2.0.4)
       nokogiri (~> 1.8, >= 1.8.4)
     parallel (1.20.1)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (4.0.6)
+    racc (1.5.2)
     rainbow (3.0.0)
     rake (13.0.3)
     rb-fsevent (0.10.4)
@@ -90,7 +92,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
-  html-proofer (~> 3.18.5)
+  html-proofer (~> 3.18.6)
   jekyll (~> 4.2.0)
   jekyll-feed (~> 0.15.1)
   jekyll-paginate (~> 1.1.0)