BREAKING: Drop Node 8 support

[feross]

Use the new fs.mkdir {recursive: true} option instead of the mkdirp dependency.

It might be too early to merge this, since Node 8 is still supported until the end of the 2019. But just sending this PR so we can merge it at some point in the future.

For: brave/brave-browser#5490

[juliangruber]

TIL about fs.mkdir(dir, { recursive: true }) :)

[mafintosh]

This is a pretty critical dep for all our p2p stuff, so this (prob) won't get merged until 8 is EOL + 1 or 2.

Future notice, requires a major bump as well

[feross]

this (prob) won't get merged until 8 is EOL + 1 or 2

@mafintosh Sounds good.

[jimmywarting]

Time to merge now?