changed 3.8 to 3.8-dev in travis.yml

Author: domdfcoding

.travis.yml

@@ -4,7 +4,7 @@ python:
   - "3.5"
   - "3.6"
   - "3.7"
-  - "3.8"
+  - "3.8-dev"
 # command to install dependencies
 install:
   - pip install -U coveralls pytest