Merge pull request #78 from enter-at/fix/marshmallow

chore(marshmallow): update to version 3.x

Author: sleistner

Pipfile

@@ -21,7 +21,7 @@ tox = ">=3.13.2"
 coverage = "*"
 wily = "*"
 jsonschema = "==3.0.2"
-marshmallow = "==2.19.5"
+marshmallow = ">=3.2.1"
 pyclean-py = "*"
 pytest-mock = "*"
 rope = "*"