Update quart from 0.6.3 to 0.6.4

Author: pyup-bot

requirements_dev.txt

@@ -9,7 +9,7 @@ sanic==0.7.0
 aiohttp==3.3.2
 tornado==4.5.3
 async_generator==1.9.0
-quart==0.6.3;python_version>="3.6"
+quart==0.6.4;python_version>="3.6"
 
 # tests
 coverage==4.5.1