Fix for support Packer 0.5.0 .

- Rename `virtualbox` to `virtualbox-iso`.
- Rename `vmware` to `vmware-iso`.
- Add `override` key to `post-processors`.
Fix iso_url to `http://ftp.riken.jp`.

- `http://ftp.iij.ad.jp/pub/linux/centos/6.4` is depreciated.
Merge pull request #3 from heavenshell/fix_for_ver0_5_0

Fix for support ver 0.5.0