update build

https://www.npmjs.org/doc/misc/npm-scripts.html

If you have to do other things before your package is used, in a way
that is not dependent on the operating system or architecture of the
target system, then use a prepublish script instead. This includes
tasks such as:

Compile CoffeeScript source code into JavaScript.
Create minified versions of JavaScript source code.
Fetching remote resources that your package will use.