Make the Console\Command and Console\Migrations\BaseCommand abstract

Following up on [Should \Illuminate\Console\Command.php be Declared `abstract`?](https://github.com/laravel/framework/issues/4273), this PR makes `Illuminate\Console\Command` and `Illuminate\Database\Console\Migrations\BaseCommand` abstract classes.

This time targeting the 4.2 branch (re: @taylorotwell's [comment on PR4372](https://github.com/laravel/framework/pull/4372#issuecomment-44036072))