switch to official slimerjs npm package

[graingert]

No description provided.

[KevinGrandon]

Thank you very much for the patch. I think we're missing a reference somewhere as the tests are failing in Travis. The main reason for the fork at this point in time is due to:

• Slow MaxVersion bumps from SlimerJS. We have business requirements to test the latest Firefox version at all times (even if SlimerJS isn't working fully). I've opened this issue to address it: Options for faster updates of MaxVersion laurentj/slimerjs#631

• Slow NPM version updates. The NPM module isn't always updated. I'd be happy to help script the update, or publish it manually should we get the Firefox version update fixed.

I would love to get rid of the fork if we can work with SlimerJS contributors to define a versioning strategy that works for us. I'm happy to help contribute to the main SlimerJS repo to make this happen.