Upgrade node-gyp from ^3.0.3 to ^6.0.1 (#264)
Upgrade node-gyp from ^3.0.3 to ^6.0.1 (#264)

Among the notable changes: it will pick `python3` over `python2` where
it exists on the system, improves Visual Studio detection and adds 
VS2019 support. For details and other breaking changes, see:

https://github.com/nodejs/node-gyp/blob/master/CHANGELOG.md#v400-2019-04-24
https://github.com/nodejs/node-gyp/blob/master/CHANGELOG.md#v500-2019-06-13
https://github.com/nodejs/node-gyp/blob/master/CHANGELOG.md#v600-2019-10-04
mafintosh/prebuild,mafintosh/prebuild,mafintosh/prebuild