DoctrineUserProvider fixed UserProvider contract usage

[torniker]

In laravel 5.1 UserProviderInterface has been removed so I changed it to the correct one from Illuminate\Contracts\Auth\UserProvider.

[GrahamCampbell]

You'd need to drop support for laravel 4 and 5.0 first...

[torniker]

I think for the repository would be best if it supports laravel 5.1 because it is announced as LTS version of laravel and I think a lot of users will use it. Of course it would be nice to have branches for laravel 4 and laravel 5.0

[GrahamCampbell]

Unfortunately, the package maintainer has already used the wrong version constraint. This package is marked as compatible with all 5.x releases, and it clearly isn't. Changing the version constraint at a later point simply isn't good enough because composer will just go and install the old version since it's marked as compatible.