[Snyk] Security upgrade html-proofer from 3.15.3 to 3.15.3

[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
	416/1000 Why? Recently disclosed, Has a fix available, CVSS 2.6	XML External Entity (XXE) Injection SNYK-RUBY-NOKOGIRI-1055008	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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[thejuliekramer]

@adborden can we merge this since tests are passing?

[adborden]

Yeppers.