Give the VM a cleaner name of "homestead"

From https://github.com/laravel/homestead/pull/95

> This is just a little cosmetic change.
If you have a lot of different VMs (with different providers like e.g. puphpet or ansible) you can increase the readability in tools like VagrantBar (https://github.com/BipSync/VagrantBar) when you have a cleaner name of homestead instead of the gibberish default naming of homestead_default_20140828141123 when displayed in the GUIs.