Add a check for the cryptography version before attempting to use it. (…

…#243)

google/auth/crypt/_cryptography_rsa.py

@@ -25,10 +25,22 @@
 from cryptography.hazmat.primitives import serialization
 from cryptography.hazmat.primitives.asymmetric import padding
 import cryptography.x509
+import pkg_resources
 
 from google.auth import _helpers
 from google.auth.crypt import base
 
+_IMPORT_ERROR_MSG = (
+    'cryptography>=1.4.0 is required to use cryptography-based RSA '
+    'implementation.')
+
+try:  # pragma: NO COVER
+    release = pkg_resources.get_distribution('cryptography').parsed_version
+    if release < pkg_resources.parse_version('1.4.0'):
+        raise ImportError(_IMPORT_ERROR_MSG)
+except pkg_resources.DistributionNotFound:  # pragma: NO COVER
+    raise ImportError(_IMPORT_ERROR_MSG)
+
 
 _CERTIFICATE_MARKER = b'-----BEGIN CERTIFICATE-----'
 _BACKEND = backends.default_backend()