Redis 4.0.0b1 has been uploaded to pip as a prerelease version. This
version drops support for Python 2 and only supports Python 3. Because
setup.py is being run, it will use the latest version of a package and
not the latest stable version (which is still 3.5.3).
Therefore, pin the redis package to version 3.5.3, so that it will work
for both Python 2 and 3.
#### How to verify it
Make sure that redis-dump-load for Python 2 builds today.
