Changed fire method to handle to meet Laravel's 5.5 requirements

[IvanMilanov]

When executing the entrust:migration command, an error is thrown on L5.5:
Method Zizaco\Entrust\MigrationCommand::handle() does not exist

Changed MigrationCommand's fire() method to handle()

[aslamsayyed]

I think better to keep fire() and call it through handle for backward compatibility.

public function handle() {
  $this->fire();
}

[dmason30]

Close this already in #840 as a backwards compatible fix.

[Zizaco]

Closed since #840 was merged