Change composer autoload type to classmap (IronMQ issue with laravel)

This issue crosses over to https://github.com/iron-io/iron_mq_php but the fix is at least needed here.  There seems to be an order dependancy issue when iron_mq_php is added as a requirement to a fresh laravel install. This results in a  composer autoload_files similar to this: 
```
		return array(
    $vendorDir . '/swiftmailer/swiftmailer/lib/swift_required.php',
    $vendorDir . '/iron-io/iron_mq/IronMQ.class.php',
    $vendorDir . '/ircmaxell/password-compat/lib/password.php',
    $vendorDir . '/laravel/framework/src/Illuminate/Support/helpers.php',
    $vendorDir . '/iron-io/iron_core/IronCore.class.php',
);
```

Since IronMQ loads before IronCore this https://github.com/iron-io/iron_mq_php/blob/master/IronMQ.class.php#L19 triggers and IronMQ doesn't get loaded.  

Long story short, changing the composer autoload type from 'files' to 'classmap' in IronCore (and IronMQ if you want) resolves this issue.