upgrade pip before installing requirements

circle.yml

@@ -3,6 +3,7 @@ dependencies:
     override:
         - rm -rf /home/ubuntu/virtualenvs/venv-*/bin/serpent
         - rm -rf /home/ubuntu/virtualenvs/venv-*/lib/python3.4/site-packages/apsw*
+        - pip install --upgrade pip
         - pip install -r requirements.txt
         - python setup.py install --with-serpent
         - python -c "import apsw; print(apsw.apswversion())"