upgrading travis' default packages

.travis.yml

@@ -5,6 +5,9 @@ services:
 python:
   - "3.6"
   - "3.7-dev"
+before_install:
+  # travis has old versions pre-installed, so upgrade them first:
+  - pip list --outdated | cut -d ' ' -f1 | xargs -n1 pip install -U
 install:
   - pip install -r requirements.txt
   - pip install pytest-cov coveralls codecov