Upgrade pip before running tests on travis

This should ensure that all tests use the same version of pip and this should hopefully fix the pypy build that is using pip 19

.travis.yml

@@ -17,8 +17,12 @@ matrix:
     - python: 3.6
       env: TOX_ENV=flake8
 
+# This should ensure that all tests use the same pip version
+before_install:
+  - python -m pip install --upgrade pip
+
 # Use tox to run tests on Travis-CI to keep one unified method of running tests in any environment
-install: 
+install:
   - pip install coverage coveralls tox
 
 # Command to run tests, e.g. python setup.py test