Update npm prepublish script to prepare

[sgobotta]

Is your feature request related to a problem? Please describe.
Related to an npm warning

Output on 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.

Describe the solution you'd like
Update the script to use prepare

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.
npm Documentation

[sgobotta]

Merged in #208