Fix CI py3.6 cryptography install from source

Author: JacobHayes

.circleci/config.yml

@@ -34,7 +34,7 @@ jobs:
           name: Build package under test and check for PyPI compliance via twine
           command: |
             . venv/bin/activate
-            pip install twine
+            pip install twine --only-binary=cryptography
             python setup.py sdist
             twine check dist/`ls dist/ | grep .tar.gz`
       - run: