Install requirements.txt in testenv

We need to install the requirements.txt to make sure we have the
correct pip version in the environment

Author: jhesketh

tox.ini

@@ -7,6 +7,7 @@ deps =
     pretend
     pytest
     pytest-capturelog
+    -r{toxinidir}/requirements.txt
 commands =
     coverage run --source pip_missing_reqs/find_missing_reqs.py \
         -m pytest --strict []