Add missing dependencies

Windows people reported that after setting up virtualenv and installing
gitflow from sources, they got an exception reporting
pkg_resources.DistributionNotFound: gitdb>=0.5.1.

This patch adds exactly this dependency to setup.py

Change-Id: Ie9d2ab51002cc9aeecd39f843406939cff6994cf
Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>