Update devDependencies to fix the npm vulnerabilities

[XhmikosR]

The semver compliant ones, i.e. no major version bump.

This fixes 68 vulnerabilities. (from 93 down to 25)

Not sure which Node.js version the contributors use, but IMO they should use >=8 to get npm 6 which has npm audit support.

[coveralls]

Coverage increased (+0.03%) to 91.713% when pulling 6e51727 on XhmikosR:xmr-npm-audit into 22831c5 on mochajs:master.

[plroebuck]

Just because a new version of a dependency has been released doesn't imply it should be updated here. Checks should be done on each individually to ensure they complied with semver rules.

[XhmikosR]

I didn't just update the version, just to update it. As you can see the dep tree is flattened a lot and many npm vulnerabilities are fixed.

That's a huge thing you should care about.

[XhmikosR]

Down to 25 vulnerabilities, from 93.

Also fixes those npm i errors coming from the zopfli wrapper, due to the assetgraph-builder update

[boneskull]

LGTM, thanks.