Upgrade to Laravel 5.1

Laravel 5.1 has been released June 9th and ofers some advantages:

• it's a LTS version;
• the documentation has been greatly improved.

This change upgrades Auth Grove to require Laravel 5.1 (and so PHP 5.5.9+).

The following instructions have been used:
http://laravel.com/docs/5.1/upgrade#upgrade-5.1.0

Authentication part will be upgraded in a follow-up change. It currently
works as it, as we use our own authentication traits forked from base.