Try to fix non-deterministic build fail on travis

This was sometimes failing with a long traceback including
`AttributeError: _DistInfoDistribution__dep_map` when trying to upgrade
pytest, which can apparently be solved by upgrading pip. We'll see.

.travis.yml

@@ -9,6 +9,7 @@ python:
   - pypy
 
 install:
+  - pip install --upgrade pip
   - pip install --upgrade pytest
   - pip install coveralls