[Snyk] Security upgrade html-proofer from 3.18.8 to 3.19.3

[snyk-io]

Snyk has created this PR to fix 1 vulnerabilities in the rubygems dependencies of this project.

Snyk changed the following file(s):

• Gemfile

⚠️ Warning

Failed to update the Gemfile.lock, please update manually before merging.

Vulnerabilities that will be fixed with an upgrade:

	Issue	Score
	Buffer Under-read SNYK-RUBY-NOKOGIRI-9789079	50

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