src: bump version to v8.0.0 for master

We have a v7.x branch which owns the v7.x versioning so master needs to move on to v8. One of the primary problems we have right now is nightly builds: https://nodejs.org/download/nightly/ they are still coming off master so the only v7.x builds you see there are from master and not from the new v7.x branch. I'd like to have v8.0.0-pre builds coming out of master and start a new nightly stream off v7.x-staging.

I don't think we're updating NODE_MODULE_VERSION at the same time here, that's going to come when we bump V8 on master right? I've already forgotten how our conversation about that played out, @TheAlphaNerd?