Update node engine to 8.12

[alanmoo]

Due to some changes in node/MacOS, it appears node >= 8.12 is required for this to run on a Mac.

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[Pomax]

I hate node-gyp so very, very, very, incredibly, much. Literally the entire world wants a native JS version, and have for years. And then when people go "okay look, fine, we've written it for you will you PLEASE start using this" they still go "hmmmmm no"