Bump node to >=8.9.x

[dwightjack]

@kino90 What about dropping support for node 6 and use node 8.9.x as the lowest required version of node? This way we could even use async/await

[kino90]

I'm actually on node 8.0.0 due to other project using dependencies that aren't compatible with node >8 (I don't remember exactly, sorry 😂)

BTW I'm also using nvm, so no problem for me!

[dwightjack]

Hm, Node >=8.0.0 will work as well. Let's use that