Merge pull request #744 from Ayrx/osx-version-check

Added check to turn off CC backend for OS X version < 10.8
diff --git a/cryptography/hazmat/bindings/commoncrypto/binding.py b/cryptography/hazmat/bindings/commoncrypto/binding.py
index 45c0eaa..ee80942 100644
--- a/cryptography/hazmat/bindings/commoncrypto/binding.py
+++ b/cryptography/hazmat/bindings/commoncrypto/binding.py
@@ -14,6 +14,7 @@
 from __future__ import absolute_import, division, print_function
 
 import sys
+import platform
 
 from cryptography.hazmat.bindings.utils import build_ffi
 
@@ -46,4 +47,5 @@
 
     @classmethod
     def is_available(cls):
-        return sys.platform == "darwin"
+        return sys.platform == "darwin" and list(map(
+            int, platform.mac_ver()[0].split("."))) >= [10, 8, 0]