fix package.json to make it work with webpack

[pgrm]

This should fix https://github.com/isteven/angular-multi-select/issues/403 - unfortunately the fix described in that issue didn't work for us, this works now. Of course this means that people need to write 2 require lines:

require('isteven-angular-multiselect/isteven-multi-select.js');
require('isteven-angular-multiselect/isteven-multi-select.css');

or

require('isteven-angular-multiselect');
require('isteven-angular-multiselect/isteven-multi-select.css');