[Snyk] Upgrade koa-helmet from 4.0.0 to 4.2.1

Snyk has created this PR to upgrade koa-helmet from 4.0.0 to 4.2.1.

merge advice

:information_source: Keep your dependencies up-to-date. This makes it easier to fix existing vulnerabilities and to more quickly identify and fix newly disclosed vulnerabilities when they affect your project.

The recommended version is 3 versions ahead of your current version.
The recommended version was released a year ago, on 2019-08-12.

The recommended version fixes:

Severity	Issue	Exploit Maturity
	Configuration Override SNYK-JS-HELMETCSP-469436	No Known Exploit

Release notes

Package name: koa-helmet

4.2.1 - 2019-08-12
- Reverts a breaking change introduced in v4.2.0 (Thanks @rahulroy9202!) which dropped node v6 support and should have been done as part of a major-release. Will re-release as part of a major version bump (v.5.0.0)
4.2.0 - 2019-06-05
- Update helmet dependency to ^3.18.0.
- Update downstream handlebars dependency to fix WS-2019-0064.
- Remove support for node 6.x due to it being out of LTS scope.
- Remove custom promisify method to use node 8's util.promisify method.
4.1.0 - 2019-03-10
- Update helmet dependency to ^3.15.1
4.0.0 - 2018-03-27
- Update helmet to ^3.12.0
- Drop node v4 support. Minimum version is Node v6.0.0

from koa-helmet GitHub release notes

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rithinch / event-driven-microservices-docker-example

[Snyk] Upgrade koa-helmet from 4.0.0 to 4.2.1 #5

Snyk has created this PR to upgrade koa-helmet from 4.0.0 to 4.2.1.