[Snyk] Security upgrade nokogiri from 1.5.0-java to 1.13.6

[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

⚠️ Warning

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

Vulnerabilities that will be fixed

With an upgrade:

Severity	Priority Score (*)	Issue	Breaking Change	Exploit Maturity
	696/1000 Why? Recently disclosed, Has a fix available, CVSS 8.2	Improper Handling of Unexpected Data Type SNYK-RUBY-NOKOGIRI-2840634	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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