Replace deep-equal with dequal

[wojtekmaj]

This PR replaces deep-equal with dequal, an alternative package with 0 dependencies (as opposed to 49: https://npmgraph.js.org/?q=deep-equal).

[wesleytodd]

We do not typically like to take package swaps for this reason. Is there any functional reason to pursue this change? In this specific case we would decline this PR anyway because the package currently supports node's older than 6. Even when we move past that hurdle, I don't see a reason to change this dependency based solely on how many transitives it brings in.

[wojtekmaj]

Since it's a devDependency, it's only going to affect the development environment. No user-facing code has been altered. I don't think any sane person would develop this package using Node.js 6.

The motivation for this change is to make this repo lighter and faster to install.

[wesleytodd]

All our dev deps need to run in CI, and CI runs in older versions. As for faster to install, can you run some benchamrks and show how much difference this makes? Please do so with both cold and warm cache.