Upgrade pip

Author: mgedmin

.travis.yml

@@ -78,7 +78,7 @@ after_success:
       # macpython 3.5 doesn't support recent TLS protocols which causes twine
       # upload to fail, so we use the system Python to run twine
       /usr/bin/python -m ensurepip --user
-      /usr/bin/python -m pip install --user twine
+      /usr/bin/python -m pip install --user -U pip twine
       /usr/bin/python -m twine check dist/*
       if [[ $TRAVIS_TAG ]]; then
         /usr/bin/python -m twine upload --skip-existing dist/*