Made attribute optional (as it is in pyOpenSSL)

See https://github.com/pyca/pyopenssl/commit/0d7e8a1af28ab22950b21afa3fd451cec7dd5fdc -- It looks like this constant isn't set on some platforms (including Raspberry Pi's libssl)
Merge pull request #44 from kronick/master

Made attribute optional (as it is in pyOpenSSL)