node-gyp shouldn't redownload the linking libraries on every install

`node-gyp install` will *always* redownload unless it has the [`--ensure` flag](https://github.com/TooTallNate/node-gyp/blob/master/lib/install.js#L81) passed to it. Atom also runs `node-gyp install` on every package install.

This has been the source of [***major*** speed problems](https://discuss.atom.io/t/why-is-apm-so-ridiculously-slow-for-me/12035) for me on my slow internet.