try to fix circle breakage on installing grpcio by upgrading pip

matches snarfed/bridgy@584f899

.circleci/config.yml

@@ -28,6 +28,7 @@ jobs:
             apt-get install -y python3-venv python3-dev
             python3 -m venv local3
             . local3/bin/activate
+            pip install -U pip
             pip install cython  # avoids grpcio install bug (search _needs_stub)
             pip install -U -r requirements.txt
             pip install coverage coveralls