Update index.js
Update index.js

Fix for reactMixin undefined error (added 'var' keyword)
Gaelan/react-mixin,shilu89757/react-mixin,dallonf/react-mixin,shilu89757/react-mixin,STRML/react-mixin,dallonf/react-mixin,Gaelan/react-mixin,brigand/react-mixin,trusktr/react-mixin,shilu89757/react-mixin,Gaelan/react-mixin,trusktr/react-mixin,dallonf/react-mixin,trusktr/react-mixin,brigand/react-mixin,brigand/react-mixin,STRML/react-mixin,STRML/react-mixin