Fix horizon pip

This resolves the following error:

{{{
NOTIFIED: [horizon | horizon venv] ********************************************
stderr: Command "tools/with_venv.sh pip install --upgrade pip>=1.4" failed.
None
stdout: Wheel installs require setuptools >= 0.8 for dist-info support.
pip's wheel support requires setuptools >= 0.8 for dist-info support.
Storing debug log for failure in /root/.pip/pip.log
venv already exists...
Installing dependencies with pip (this can take a while)...
}}}