Browser-compat-data: Raise minimum node version to 8 in April 30, 2019

[ExE-Boss]

Questions before I merge this:

• Do we also want to lower the minimum node version now?
• If so, do we also check what minimum node versions our (dev)Dependencies have?

_Originally posted by @Elchi3 in https://github.com/mdn/browser-compat-data/pull/3442#pullrequestreview-204797517_

[jwhitlock]

We should not support a version of node that is not supported by the Node Foundation. Even if we're compatible with earlier versions, we should require node 6, and raise that to 8 in April 2019.

[Elchi3]

Thanks John, I agree.

[vinyldarkscratch]

Well, now it is April, and soon, the EOL of v6 of Node. I strongly agree -- on April 30th, 2019, let us raise the minimum Node version to 8.