[Snyk] Security upgrade html-proofer from 3.19.4 to 4.0.0

[VonnyJap]

Snyk has created this PR to fix 8 vulnerabilities in the rubygems dependencies of this project.

Snyk changed the following file(s):

• Gemfile
• Gemfile.lock

Vulnerabilities that will be fixed with an upgrade:

	Issue	Score
	Heap-based Buffer Overflow SNYK-RUBY-NOKOGIRI-7164639	696
	Stack-based Buffer Overflow SNYK-RUBY-NOKOGIRI-8732769	671
	Use After Free SNYK-RUBY-NOKOGIRI-8732779	671
	Use After Free SNYK-RUBY-NOKOGIRI-9510795	576
	Use After Free SNYK-RUBY-NOKOGIRI-6228056	524
	Use After Free SNYK-RUBY-NOKOGIRI-9510789	469
	Buffer Under-read SNYK-RUBY-NOKOGIRI-9789079	426
	Cross-site Scripting (XSS) SNYK-RUBY-NOKOGIRI-8453714	319

---

[!IMPORTANT]

• Check the changes in this PR to ensure they won't cause issues with your project.
• Max score is 1000. Note that the real score may have changed since the PR was raised.
• This PR was automatically created by Snyk using the credentials of a real user.

---

Note: You are seeing this because you or someone else with access to this repository has authorized Snyk to open fix PRs.

For more information: 🧐 View latest project report 📜 Customise PR templates 🛠 Adjust project settings 📚 Read about Snyk's upgrade logic

---

Learn how to fix vulnerabilities with free interactive lessons:

🦉 Use After Free 🦉 Cross-site Scripting (XSS)