Add Sortable as dependency

[refacrot]

Hey,

I had Sortable plugin for a long time in my project but it failed on the new version of React. I added this mixin and got following error:

ERROR in ./~/react-mixin-sortablejs/react-mixin-sortable.js
Module not found: Error: Cannot resolve 'file' or 'directory' ./Sortable in /Users/JDS/Git/mobile/node_modules/react-mixin-sortablejs
 @ ./~/react-mixin-sortablejs/react-mixin-sortable.js 13:2-33

I made small fixes so please review it and merge if it looks good.
Feel free to criticize my changes.
Thanks.