Upgrade execa to v4 and require Node.js 10

[artursvonda]

Since Node v8 is being sunset at the end of the year, used this opportunity to jump on latest execa.

[sindresorhus]

I don’t personally like execa.command. I prefer the explicitly of an array of arguments. How is it easier to mock?

[artursvonda]

It was easier to mock property of execa instead of execa itself based on the code that already was there. I actually didn't try mocking execa.default to see if that would work. I think if we want mock the whole import (instead of just property like we do now), it requires different approach.
I'm open to suggests if you have any. When I have a moment to spare, I could also look into proper mocking of imports if you think the current solution will not work.

[sindresorhus]

When I have a moment to spare, I could also look into proper mocking of imports if you think the current solution will not work.

Yes, I would prefer that. Could use sinon or proxyquire.

[artursvonda]

Rebased and now it's a lot cleaner.

[sindresorhus]

Great. Thanks :)