Fix invalid cast from ASN1_TIME to ASN1_GENERALIZEDTIME (#612)

* Fix invalid cast from ASN1_TIME to ASN1_GENERALIZEDTIME, which ends up with an error with LibreSSL.

* Require cryptography >= 1.9
diff --git a/setup.py b/setup.py
index 97aa209..4f30804 100755
--- a/setup.py
+++ b/setup.py
@@ -96,7 +96,7 @@
         package_dir={"": "src"},
         install_requires=[
             # Fix cryptographyMinimum in tox.ini when changing this!
-            "cryptography>=1.7",
+            "cryptography>=1.9",
             "six>=1.5.2"
         ],
     )