Rename VagrantFile to Vagrantfile

Despite https://docs.vagrantup.com/v2/vagrantfile/ stating that "casing doesn't matter", the issue at https://github.com/hellais/TAILS-OSX/issues/50 suggests otherwise. I also reproduced this by using a case sensitive formatted disk image and was able to test the fix this way.

I also created an issue on the vagrant github for this bug too: https://github.com/mitchellh/vagrant/issues/4907
Changed links to Tails v1.2.1 ISO and signature

1.2 was no longer available for download.

Changed links to get v1.2.1