Bundler friendly (and ES6 compatible) requires

I removed dynamically resolved paths with static once. First one cannot be resolved automatically by CJS bundlers like [Webmake](https://github.com/medikoo/modules-webmake/) or [Browserify](http://browserify.org/).
Additionally upcoming ES6 modules syntax won't support dynamic imports, so it's first step to make this package compatible with upcoming native format.