Upgrade template to conform with Packer v0.5.0

Version 0.5.0 introduced a backward incompatiblity around builder

- `virtualbox` was renamed to `virtualbox-iso`
- `vmware` was renamed to `vmware-iso`
Upgrade Ubuntu to 12.04.4

Use latest available version of Ubuntu LTS for building.
Merge pull request #9 from luislavena/upgrade-ubuntu

Upgrade ubuntu to 12.04.4