The secret key that encrypts the backups should not be world readable.
Fix permissions of backup secret according to Josh's comment at
https://github.com/mail-in-a-box/mailinabox/pull/150#issuecomment-53120156
Fix wrong permissions of backup secret. Pyhton 3 needs octal permissions.
Merge pull request #150 from hjjg/secretkeyfix

The secret key that encrypts the backups should not be world readable.