Support React v15

[cheapsteak]

error:

Cannot find module 'react/lib/Object.assign' from 'C:\Users\projects\react-template\node_modules\react-transition-group-plus\src'

Should replace

var assign = require('react/lib/Object.assign');

at https://github.com/cheapsteak/react-transition-group-plus/blob/master/src/ReactTransitionGroupPlus.js#L18

with

var assign = require('object-assign');

via https://www.npmjs.com/package/object-assign

(will need to add a dependency)

[cheapsteak]

closed by https://github.com/cheapsteak/react-transition-group-plus/commit/38a55af0cd73f799d1d6ccfbe50bf5672e0d4b80