Try to get coverage working on Travis.

.travis.yml

@@ -1,12 +1,20 @@
 language: python
+
 python:
     - "3.6"
     - "3.7"
     - "3.8"
+
 install:
     - pip install -U pytest hypothesis pytest-cov coveralls attrs
     - pip install .
+
+    # Can't imagine why this would matter, but see:
+    # https://thomas-cokelaer.info/blog/2017/01/pytest-cov-collects-no-data-on-travis/
+    - touch tests/__init__.py
+
 script: 
     - pytest tests --cov wellmap
+
 after_success:
     - coveralls