Update Bundler sandbox to Vagrant v1.6.5

Skipping v1.6.4 as that release completely broke private networking for RedHat
VMs.
Better comment documentation for Gemfile.local

Also added `Gemfile.lock` to `.gitignore`.
Don't overwrite nightly version and download_root

When using nightly builds, allow users to explicitly set `version` and
`download_root`.
added PE 3.7.0 to example file, kept 3.4.0 for transitional purposes
Update vms.yaml.example