dillon1000/react

Update blog post which creates unhandled promise rejection (#9668)

* Update 2015-12-16-ismounted-antipattern.md

In case anybody else stumbles across this old blog post, I wanted to submit a patch to help with unhandled rejections.  

`#then` and `#catch` each return new Promise instances, so here we actually create two new promises (that aren't assigned).  If the argument promise to `#makeCancelable` rejects, the promise created by `#then` will be an unhandled rejection, which in Node 7 will be an uncaught error.  

By using the second argument of `#then` to handle rejections instead, we don't need to worry about the runtime finding any unhandled rejections here.

* Style updates

* Add update notice