Specify minimal version for cryptography package

Otherwise if pysaml is installed with an older release of cryptography package it would fail with
AttributeError: '_RSAPrivateKey' object has no attribute 'sign'

Author: AmbientLighter

setup.cfg

@@ -48,7 +48,7 @@ scripts =
     tools/merge_metadata.py
     tools/parse_xsd2.py
 install_requires =
-    cryptography
+    cryptography >= 1.4
     defusedxml
     future
     pyOpenSSL