Fixes #784 -- bump minimum cryptography version (#785)

diff --git a/setup.py b/setup.py
index 29c11f4..3728795 100755
--- a/setup.py
+++ b/setup.py
@@ -95,7 +95,7 @@
         package_dir={"": "src"},
         install_requires=[
             # Fix cryptographyMinimum in tox.ini when changing this!
-            "cryptography>=2.2.1",
+            "cryptography>=2.3",
             "six>=1.5.2"
         ],
         extras_require={