Drop support for v0.12

[nfischer]

According to the NodeJS LTS schedule, v0.12 support was dropped by Node in December 2016. This means it's probably reasonable to drop the support in our v0.8 release.

This has some benefits for us:

• we can cut v0.11 and v0.12 from our Travis builds for faster CI (maybe even iojs too, which sometimes has false negatives)
• we can significantly trim down on the code inside src/exec.js (there's a whole v0.10-specific else-branch)
• we can clean up tests
• we can trim down src/tempdir.js a bit as well
• we can remove our Object.assign() polyfill (if we remove iojs as well)

@freitagbr what are your thoughts on dropping v0.12 support? What about dropping iojs support? We'll get the biggest CI benefits if we go with iojs, but I don't know if many people still rely on it (my guess is iojs users have moved on to v4+).

[freitagbr]

I would be in favor of dropping support for iojs. I don't see it being widely used in production or even development, and like you said, most iojs users should at least be on v4 or later by now.

I know usage of v0.12 is pretty common still, though dropping support for v0.12 and below is probably fine as long as this lands in v0.8+ only.

Just make sure that "engines": { "node": ">=4" } ends up in package.json.

[nfischer]

Ok, I can take this up