Revert "Remove full-disk-encryption requirement"

This reverts commit 0afcaf21c1cd5afe2feb643b7d7d761032fab07b.

New OSX builds from IT now use FV2 instead of PGP. Because Boxen requires
Mountain Lion there's a good chance, one way or another, everyone has FV2.
Thus we should enforce this sensible default.
Merge pull request #4 from alphagov/enforce_fde

Revert "Remove full-disk-encryption requirement"