Accept VirtualBox release candidate version

With this fix, packer get version for example 4.3.14_RC1 from 4.3.14_RC1r94870.

I also made the minimal fix at 
https://github.com/hnakamur/packer/compare/accept_virtualbox_release_candidate_version_with_minimal_change
but I think this pull requst is easier to understand.

You can try this regexp at
http://play.golang.org/p/9DizYfSy7Y