travis: Add nightly build again, but allow it to fail

.travis.yml

@@ -6,6 +6,10 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
+  - "nightly"
+matrix:
+  allow_failures:
+    - python: "nightly"
 
 install:
   - pip install coveralls tox-travis