travis: add Python 3.8

- allow failures for now

.travis.yml

@@ -6,6 +6,7 @@ python:
 - "3.4"
 - "3.5"
 - "3.7"
+- "3.8"
 
 jobs:
   fast_finish: true
@@ -13,6 +14,7 @@ jobs:
   - python: "3.4"
   - python: "3.5"
   - python: "3.7"
+  - python: "3.8"
 
 env:
 - PIPENV_VERBOSITY=-1