ci: add osx to build matrix

[b-admike]

This PR looks to add osx to our travis build matrix. It gives us more platform coverage, but is also used as a way to fix build issue arising from #948.

Checklist

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• Related API Documentation was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in packages/example-* were updated

[bajtos]

Nice! Please wait for CI results to become green before landing.

On an unrelated note, I noticed that we are running our commit-linter on Node.js 6.x, can we eventually switch to 8.x instead?

[virkt25]

Once 6 is dropped from the node_js matrix, 8 will become the default for commit-linter.