Added a mailmap to clean up authors on rubygems

A mail map combines authors and changes their names so that it looks correct with `git shortlog`. I used it to clean up the ruby gems author list.
> Is this correct? This would list you as "joshua-anderson", not "Joshua Anderson".

I want to be `joshua-anderson` as that's my rubygems account.

> Also, I think you can simplify the rules a bit:

I didn't know that you could list just email's. Thanks for the tip!