Update MigrationCommand.php

Removed array dereferencing for PHP 5.3 support
Update composer.json

Updated composer to only require PHP 5.3

Strictly speaking, 5.4 is required for the HasRole trait, although dropping back to requiring 5.3 allows people to choose to work around the trait if they wish to, resulting in Entrust working with 5.3
Merge pull request #56 from JoeChilds/master

Update MigrationCommand.php. Improving possible php 5.3 support