Update README.md

These lines in the source:

    child = child_process.spawn(path.resolve('./node_modules/.bin/protractor'), args, {
        stdio: 'inherit'
    }).on('exit', function() {
        if (child) {
            child.kill();
        }
        if (stream) {
            stream.emit('end');
        }
    });

depend on the current working directory, so if protractor isn't directly installed as a dependency, this won't work.