Expand travis.yml build platforms

[Aerijo]

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Issue or RFC Endorsed by Atom's Maintainers

na

Description of the Change

Updates Travis CI to build and mac and windows too, as well as maximising parallel compilation and removing unnecessary config values.

It's good to build and test for these platforms, as native code (especially a third party module) may have platform specific issues.

Alternate Designs

It technically works as is, but this should make it catch more errors

Possible Drawbacks

Goes from 4 builds to 12; this can be cut to 9 if Node 6 support is dropped (Atom is on 10, and NodeJS is about to drop support for version 8).

Verification Process

Tests on a fork, and this PR itself. I anticipate an error in Node 6 with Windows. (Edit: This is the only failing job).

Release Notes

• Expand Travis CI to test macos and windows too

[Aerijo]

Could remove the windows build from Travis, and keep relying on Appveyor instead. This would cut Travis down to the 8 jobs between Linux and macos, which can be cut to 6 builds if Node 6 is dropped.