[Snyk] Security upgrade html-proofer from 3.18.5 to 3.19.3

[choony1]

Snyk has created this PR to fix 2 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
	Use After Free SNYK-RUBY-NOKOGIRI-9510795	648
	Use After Free SNYK-RUBY-NOKOGIRI-9510789	541

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[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.