Replace PhantomJS with Headless Chrome for testing

[bryanmikaelian]

This PR removes PhantomJS in favor of Headless Chrome. PhantomJS is no longer maintained and, as such, we cannot utilize ES6 features in our tests. This is part of a long-term goal to deprecate some third modules in favor of native functions (i.e. Object.assigns versus defaults)

For context, I simply followed the instructions here https://developers.google.com/web/updates/2017/06/headless-karma-mocha-chai

Test plan

Testing not required because this is a dev-only change.

Release plan

New version is not required because this is a dev-only change.

Checklist

• Thorough explanation of the issue/solution, and a link to the related issue
• CI tests are passing
• Unit tests were written for any new code
• Code coverage is at least maintained, or increased.

[pooyaj]

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