Use pip download to prepare TeamCity rather than the removed pip install --download

Author: adamtheturtle

prep_teamcity

@@ -33,7 +33,7 @@ pip install -U pip
 pip install wheel
 
 # Download distro independent artifacts
-pip install --download wheelhouse $DIR/ext/dcos-image
+pip download -d wheelhouse $DIR/ext/dcos-image
 
 # Make the wheels, they will be output into the folder `wheelhouse` by default.
 pip wheel --wheel-dir=wheelhouse --no-index --find-links=wheelhouse $DIR/ext/dcos-image