Tell pip to upgrade attrs...

Don't know why it can't do this on its own...

.travis.yml

@@ -4,7 +4,7 @@ python:
     - "3.7"
     - "3.8"
 install:
-    - pip install -U pytest hypothesis pytest-cov coveralls
+    - pip install -U pytest hypothesis pytest-cov coveralls attrs
     - pip install .
 script: 
     - pytest tests --cov wellmap