Store PACKER_VARS as list

To add additional PACKER_VARS in the future, make a list of packer vars and combine them with a '-var ' prefix.
Merge pull request #32 from dol/patch-1

Store PACKER_VARS as list
Version 1.0.9 published to VagrantCloud
Merge branch 'master' of github.com:box-cutter/ubuntu-vm
Bump version
Merge remote-tracking branch 'upstream/master'

Merged with upstream to upgrade VirtualBox Guest Addition to 4.3.20