Update cryptography from 2.6.1 to 2.7

Author: pyup-bot

requirements_dev.txt

@@ -3,7 +3,7 @@ pip==19.1  # pyup: update minor
 
 bumpversion==0.5.3  # pyup: update minor
 wheel==0.33.0  # pyup: update minor
-cryptography==2.6.1  # pyup: update minor
+cryptography==2.7  # pyup: update minor
 psycopg2-binary==2.8.1  # pyup: update minor
 sanic==19.3.1  # pyup: update minor
 aiohttp==3.5.0  # pyup: update minor