Update cryptography from 2.4.1 to 2.5 (#422)

Author: pyup-bot

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==2.4.1  # pyup: update minor
+cryptography==2.5  # pyup: update minor
 psycopg2-binary==2.7.6.1  # pyup: update minor
 sanic==18.12.0  # pyup: update minor
 aiohttp==3.5.0  # pyup: update minor