Vagrantfile: 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>
Vagrantfile: Added a check to ensure that versions older than 1.4.0 also return a version requirement message.

Also implemented the same changes in the demo Vagrantfile.

Tested with vagrant 1.3.0, 1.4.3, and 1.6.5.

Signed-off-by: John Zila <john@jzila.com>
Merge pull request #336 from jzila/issue-335

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