Bump marshmallow from 2.19.5 to 2.20.0 (#36)

dependabot-preview[bot] · xen0l · commit eebba080d475 · 2019-08-11T15:58:07.000+02:00
diff --git a/requirements/requirements.txt b/requirements/requirements.txt
@@ -1,4 +1,4 @@
 boto3~=1.9.205
-marshmallow==2.19.5
+marshmallow==2.20.0
 PyYAML==5.1.2
 requests==2.22.0
