Add getLastAttempted method to Auth\Guard class.


*Sorry, I accidentally submitted my last pull request against the Master branch instead of the 4.1 branch.*

---

I am trying to implement a legacy password upgrader in my current application.
If Auth::attempt() fails, I want to compare a legacy hash of the entered password against the password in the database. If it is a legacy password, it gets updated to a new hashed password in the database.

As it currently stands there is no way to get the last retrieved user from the Auth class, as $lastAttempted is protected and has no getter method. This forces me to either extend the Guard class, or re-retrieve the user from the database.

This pull request solves this issue by implementing a getter method for the $lastAttempted property.