Remove unecessary pip upgrade step

.travis.yml

@@ -17,10 +17,6 @@ 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:
   - pip install coverage coveralls tox