initial x509 version support

Adds enum34 as a dependency
diff --git a/setup.py b/setup.py
index 3e2ab3e..ead5ec4 100644
--- a/setup.py
+++ b/setup.py
@@ -36,6 +36,7 @@
 
 requirements = [
     CFFI_DEPENDENCY,
+    "enum34",
     "pyasn1",
     SIX_DEPENDENCY,
     SETUPTOOLS_DEPENDENCY