allow boxen runs during fde-ing

On older Mac Books with large spinny drives it can sometime take up to many several hours for a disk to be fully encrypted. Having someone wait that time before they can get rocking with Boxen is a little silly. 

[From the Apple Man pages from fdesetup](http://developer.apple.com/library/Mac/documentation/Darwin/Reference/ManPages/man8/fdesetup.8.html) an Exit Status of `2` indicates that FDE is *ON* just busy.

However one side note, if you turn FDE *OFF* the Exit Status will return `2` while the disk is decrypting, but all subsequent Boxen runs will fail unless the `--no-fde` flag is passed.