Make npm package more portable/friendly

I'm the maintainer of the [karma-chai-sinon](https://www.npmjs.org/package/karma-chai-sinon) npm package and it'd be great if you'd merge this request so everyone (my company included) could use your package flawlessly. 

Currently many developers, myself included, get dependency errors because your package.json has very strict rules.

This pull request modifies your package.json and index.js so it's more portable/friendly to different environments.

Thank you in advance!