Rescue encryptor errors

This change set handles the new exception classes introduced in https://github.com/attr-encrypted/encryptor/pull/13 . This should allow clients of AttrEncrypted to rescue any currently known exception coming out of Encryptor (and by extension OpenSSL) in a sane manner. I've had to alter the `Gemfile` to point at the updated `Encryptor`, we'll probably want to remove that once this is merged or as part of the merge.