travis: use newer pip that understands python_requires

(should fix build on Python 3.3 because of too-new tox)

.travis.yml

@@ -26,7 +26,7 @@ python:
 
 install:
     # Optimisation: build requirements as wheels, which get cached by Travis
-    - pip install "pip>=7.0" wheel
+    - pip install "pip>=9.0" wheel
     - pip install "setuptools>=38"
     - pip install tox-travis
     - pip install flake8