Set Ubuntu Raring cloud box `--ostype=Ubuntu_64` to fix virtualbox boot

For Mac OS 10.6 using Virtualbox 4.2.18 and Vagrant 1.3.5, booting the current Ubuntu 13.04 (Raring) cloud image fails because the box is set to 32-bit Ubuntu by default. Thus, we manually specify the OS type to Ubuntu 64-bit for the Virtualbox provider. This bug may be specific to my platform, but the fix is innocent enough to generalize to all platforms.