Test marshmallow 3.0.0rc6

Exclude testing version 3 with python 2

Author: SVilgelm

.travis.yml

@@ -3,15 +3,20 @@ cache: pip
 branches:
   only:
   - master
-  - /^\d+\.\d+(\.\d+)?$/
+  - /^v\d+\.\d+(\.\d+)?$/
 
 python:
   - "3.6"
   - "2.7"
 
 env:
   - MARSHMALLOW_VERSION=2.19.2
-  - MARSHMALLOW_VERSION=3.0.0rc5
+  - MARSHMALLOW_VERSION=3.0.0rc6
+
+matrix:
+  exclude:
+    - python: "2.7"
+      env: MARSHMALLOW_VERSION=3.0.0rc6
 
 install:
   - pip install marshmallow==$MARSHMALLOW_VERSION