Remove prepublish

[IgnusG]

While installing dependencies I received this message:

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.

prepare is already run after prepublish so the latter one could be removed.

· prepare:  Run  both  BEFORE  the  package is packed and published, and on local npm install without any arguments (See below). This is run
         AFTER prepublish, but BEFORE prepublishOnly.