add python-requests to README
If we don't have python-requests then we can't import
collect_atomic_vars from atomic_vars.py and it will silently
not collect any atomic vars.
/home/vagrant/fedora-websites/build.d/build.py(55)<module>()
-> from atomic_vars import collect as collect_atomic_vars
(Pdb)
ImportError: 'No module named requests'