Return transition object on successful transition

At first I wanted to ask if there're any reasons to return 'true' instead of the transition object, but it was so easy to change, so I went with a PR. 

It's easy to use history.last, but can lead to race conditions, so i think it's a good idea to return exactly what has been created by the transition. 

I'll be happy to change the readme if you decide this change is fine. Great gem btw! :heart: Thanks!