[Snyk] Security upgrade rack from 3.1.4 to 3.1.5

[mefellows]

This PR was automatically created by Snyk using the credentials of a real user.


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

Snyk changed the following file(s):

• example/zoo-app/Gemfile

⚠️ Warning

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

Vulnerabilities that will be fixed with an upgrade:

	Issue	Score
	Regular Expression Denial of Service (ReDoS) SNYK-RUBY-RACK-7416898	631

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• This PR was automatically created by Snyk using the credentials of a real user.

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🦉 Regular Expression Denial of Service (ReDoS)