vagrant: Added a version filter to the Vagrantfile to require at least vagrant 1.6.0

Manually tested the change due to the need to test with multiple vagrant
versions, which are external to this project.

Fixes #335

Signed-off-by: John Zila <john@jzila.com>
`Vagrant.require_version` was added in [v1.4.0](https://github.com/mitchellh/vagrant/blob/master/CHANGELOG.md#140-december-9-2013) so it may be worth rescuing `NoMethodError` in case the version is older than that:

```ruby
begin
  Vagrant.require_version ">= 1.6.0"
rescue NoMethodError
  $stderr.puts "This Vagrantfile requires Vagrant version >= 1.6.0"
  exit 1
end
```