Upgrades virtual hardware version

Version 4 is too low. Some VMware products even crash trying to
upgrade it to a greater version (VMware Fusion 6 Pro). Having at
least 7 will allow us to use some modern features in most VMware
products, such as enabling vmxnet3 virtual network adapters or adding
much more memory and cpu cores to virtual machines.
Merge pull request #338 from c4milo/vmware-vhardware-version-upgrade

Upgrades virtual hardware version to a more reasonable minimum default
Merge branch 'master' of https://github.com/coreos/scripts