travis: ensure test requirements are installed and up-to-date

Particularly: update pip so PEP 518 support is available.

.travis.yml

@@ -43,12 +43,14 @@ jobs:
 cache: pip
 
 install:
-# ensure we have recent pip/setuptools
-- pip install --upgrade pip setuptools
+
+# ensure we have recent pip/setuptools/wheel
+- pip install --disable-pip-version-check --upgrade pip setuptools wheel
 # need tox to get started
-- pip install tox tox-venv
+- pip install --upgrade tox tox-venv
 
 # Output the env, to verify behavior
+- pip freeze --all
 - env
 
 # update egg_info based on setup.py in checkout