Suggest similar name for useless assignment

![screen shot](https://cloud.githubusercontent.com/assets/83656/4019677/72323eea-2a81-11e4-94ea-97b7bbe7ee9c.png)

Actually this idea was generated by Matz :).

A few weeks ago he appeared on [Rebuild](http://rebuild.fm/53/) podcast (mainly in Japanese) as a guest and talked about rough ideas for post-Ruby-2.2. In the show he said it would be nice if there are some sort of features that allow us to find bugs quickly. Also, with a laugh he said how about adding a message like “This wrong variable you're using is similar to another variable in the same scope.” to error messages for typos.

Though I'm not sure how serious he was and whether it will really be implemented in MRI in the future since the idea was only in his mind and hadn't yet discussed in the core team, I thought it was feasible in RuboCop so I robbed the idea and implemented it :grin:.