Bump html-proofer from 3.18.6 to 3.18.8

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.18.6 to 3.18.8.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.18.6...v3.18.8)

Gemfile

@@ -10,6 +10,6 @@ gem 'jekyll-feed', '~> 0.15.1'
 gem "jekyll-sitemap", "~> 1.4"
 
 group :test do
-    gem 'html-proofer', '~> 3.18.6'
+    gem 'html-proofer', '~> 3.18.8'
     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 (3.18.6)
+    html-proofer (3.18.8)
       addressable (~> 2.3)
       mercenary (~> 0.3)
       nokogumbo (~> 2.0)
@@ -91,7 +91,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
-  html-proofer (~> 3.18.6)
+  html-proofer (~> 3.18.8)
   jekyll (~> 4.2.0)
   jekyll-feed (~> 0.15.1)
   jekyll-paginate (~> 1.1.0)