Merge pull request #391 from fantix/pyup-update-cryptography-2.3.1-to-2.4.1

Update cryptography to 2.4.1

Author: wwwjfy

requirements_dev.txt

@@ -3,7 +3,7 @@ pip==18.1  # pyup: update minor
 
 bumpversion==0.5.3  # pyup: update minor
 wheel==0.32.2  # pyup: update minor
-cryptography==2.3.1  # pyup: update minor
+cryptography==2.4.1  # pyup: update minor
 psycopg2-binary==2.7.6.1
 sanic==0.8.3
 aiohttp==3.4.4