Drop Node.js 6, modernize code, return Promise from async function

[coreyfarrell]

• Implement with async functions
• Use util.promisify
• Replace var with const or let
• Use arrow functions for callbacks
• Update Travis testing matrix

Fixes #33

[isaacs]

This is great! Unfortunately, I landed a few PRs before getting to this one, and it's full of merge conflicts.

Would you mind terribly rebasing onto latest master? If that's too much of a pita, I can port it myself pretty soon.

[coreyfarrell]

I can probably rebase tomorrow. Could you please look at #33 (comment) let me know what you think about the Promise option?

[isaacs]

Go ahead and drop it in the next semver major. Just use and return native promises.

[coreyfarrell]

OK rebase is done.

One note, the code to get tmpfile into the callback is a little odd now that we the async work is done in a separate function. Would it make sense to have a callback option for reporting creation of the tmpfile:

if (options.tmpfileCreated) {
  options.tmpfileCreated(tmpfile)
}

This would allow tmpfileCreated to be reported as soon as it's created rather than waiting until completion. It would also allow the sync method to report tmpfile even if a failure occurred after the tmpfile was created.

CC @novemberborn

[isaacs]

So far, temp file reporting has never been published in a release, so from my pov, we're free to implement it however. Reporting what temp file was created after it's already been deleted is definitely a bit strange.

Yes, a callback option here makes more sense, imo. Let's get that in and then cut a v3 release.