Update package.json: modify postinstall script

This will fix issue for windows paths. Node-scripts are adding ./node_modules/.bin to the path by default.
What's your version of npm (`npm -v`)? Try updating npm (`npm install -g npm`), because npm by defaults adds `./node_modules/.bin` folder to $PATH, so npm-scripts can use executables without this dance, which comes out to not work on windows (wrong separator).