Update greenlet from 0.4.12 to 0.4.13

Author: pyup-bot

requirements.txt

@@ -21,7 +21,7 @@ flake8==3.5.0
 funcsigs==1.0.2
 future==0.16.0
 gevent==1.2.2
-greenlet==0.4.12
+greenlet==0.4.13
 idna==2.6
 imagesize==0.7.1
 ipaddress==1.0.19