[Snyk] Fix for 1 vulnerabilities

[snyk-bot]

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

Changes included in this PR

• Adding or updating a Snyk policy (.snyk) file; this file is required in order to apply Snyk vulnerability patches. Find out more.

Vulnerabilities that will be fixed

With a Snyk patch:

Severity	Issue
	Command Injection SNYK-JS-TREEKILL-536781

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[codecov-io]

Codecov Report

Merging #350 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #350   +/-   ##
=======================================
  Coverage   80.67%   80.67%           
=======================================
  Files          51       51           
  Lines        2303     2303           
  Branches      571      571           
=======================================
  Hits         1858     1858           
  Misses        445      445

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[ksaaskil]

This patches tree-kill used by concurrently, which is included as a dependency in unmock-server. It's however not used anywhere so I'll delete the dependency in https://github.com/unmock/unmock-js/pull/351 instead.