changed download url arch linux to 2014-03-01

Hello,

Version 2013.12.01 got a 404 so i updated it to the latest version

==> virtualbox-iso: Gracefully halting virtual machine...
==> virtualbox-iso: Preparing to export machine...
    virtualbox-iso: Deleting forwarded port mapping for SSH (host port 2822)
==> virtualbox-iso: Exporting virtual machine...
==> virtualbox-iso: Unregistering and deleting virtual machine...
==> virtualbox-iso: Running post-processor: vagrant
==> virtualbox-iso (vagrant): Creating Vagrant box for 'virtualbox' provider
    virtualbox-iso (vagrant): Copying from artifact: output-virtualbox-iso/packer-virtualbox-iso-disk1.vmdk
    virtualbox-iso (vagrant): Copying from artifact: output-virtualbox-iso/packer-virtualbox-iso.ovf
    virtualbox-iso (vagrant): Renaming the OVF to box.ovf...
    virtualbox-iso (vagrant): Compressing: Vagrantfile
    virtualbox-iso (vagrant): Compressing: box.ovf
    virtualbox-iso (vagrant): Compressing: metadata.json
    virtualbox-iso (vagrant): Compressing: packer-virtualbox-iso-disk1.vmdk
Build 'virtualbox-iso' finished.

==> Builds finished. The artifacts of successful builds are:
--> virtualbox-iso: 'virtualbox' provider box: packer_virtualbox-iso_virtualbox.box