[Snyk] Security upgrade poltergeist from 1.18.1 to 1.18.1

[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	Issue	Breaking Change	Exploit Maturity
	Regular Expression Denial of Service (ReDoS) SNYK-RUBY-WEBSOCKETEXTENSIONS-570830	No	Proof of Concept

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

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[hugocorbucci]

This already got handled in #292