[Snyk] Security upgrade html-proofer from 3.19.2 to 3.19.2

[snyk-bot]

Snyk has created this PR to fix one or more vulnerable packages in the `rubygems` dependencies of this project.

Changes included in this PR

• Changes to the following files to upgrade the vulnerable dependencies to a fixed version:
  • Gemfile
  • Gemfile.lock

Vulnerabilities that will be fixed

With an upgrade:

Severity	Priority Score (*)	Issue	Breaking Change	Exploit Maturity
	661/1000 Why? Recently disclosed, Has a fix available, CVSS 7.5	XML External Entity (XXE) Injection SNYK-RUBY-NOKOGIRI-1726792	No	No Known Exploit

(*) Note that the real score may have changed since the PR was raised.

Check the changes in this PR to ensure they won't cause issues with your project.

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[jsf9k]

The Travis builds are failing because Travis is no longer as free (as in $$) as it once was. We have a certain number of free credits to use, and they are usually all gone by the end of the month.

[h-m-f-t]

at 3.19.3, this is old