V6: Removing support for NodeJS v6

[SimonSchick]

Node v6 is approaching the end of it's long term support window and will be end of life on 4/30.

We will no longer be supporting node v6 and you will not be able to use sequelize v6 in node v6 without the use of transpilers and polyfills.

[SimonSchick]

See related PR https://github.com/sequelize/sequelize/pull/10739/commits

[janmeier]

👍

[gabegorelick]

Node 8 reaches end of life December 31, 2019. Should support for that be dropped too?