Use Electron to run browser tests with Travis

[josephfrazier]

This makes the automated testing applied to pull requests a little more comprehensive. A couple caveats:

• This doesn't work on AppVeyor
• Most of the browser-specific tests are skipped

I'm hoping both of these can be resolved later, but this seems like a worthwhile change regardless. Here are example builds from my personal repo: Travis, AppVeyor

[feross]

Nice work! I see a one way we can improve this further:

• Run sauce tests locally. With this PR, these are only run when certain environment variables are set, but locally most people their credentials in .zuulrc. Locally can we just run all 3: node, electron, and sauce labs?

[josephfrazier]

Great idea! I'll open a PR for that.

[lock]

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