Update html-proofer requirement from 3.18.6 to 3.18.8

Updates the requirements on [html-proofer](https://github.com/gjtorikian/html-proofer) to permit the latest version.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](gjtorikian/html-proofer@v3.18.6...v3.18.8)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile

@@ -13,4 +13,4 @@ gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.1.0'
 gem 'thread_safe', '0.3.6'
 gem 'epubcheck-ruby', '4.2.4.0'
-gem 'html-proofer', '3.18.6'
+gem 'html-proofer', '3.18.8'