"prepublish" script ignored in favor of "prepare" script in future npm

[billiegoose]

$ npm install
npm WARN prepublish-on-install As of npm@5, prepublish scripts are deprecated.
npm WARN prepublish-on-install Use prepare for build steps and prepublishOnly for upload-only.
npm WARN prepublish-on-install See the deprecation note in npm help scripts for more information.

Probably my smallest PR ever.

[jvilk]

Does this make dist run twice during npm install and npm publish? It doesn't look like there's any way around it, and the documentation is quite confusing, so I'll merge this.

[billiegoose]

I don't think it ran twice on npm install. I wouldn't know whether it runs twice on npm publish. I'm sure we'll hear more noise about this as npm6 and higher come along.