adding information about tox and virtualenv

There's currently an issue with the latest version of tox and
this is keeping people from running the tests. Installing the
previous version is a workaround until the problem with tox
is solved

Change-Id: I3d8a842038f6265daf42c5624ef209bf248570a6
Signed-off-by: Thiago da Silva <thiago@redhat.com>