Remove tox 2.1.1 requirement in README.md

In order to support new tox syntax used in tox.ini, it is required to
use tox version 2.3.1 or greater.

Resolves openbmc/openbmc-test-automation#73

Change-Id: I36e2e891ccd93e795cd29310ed042522e0b68754
Signed-off-by: Jay Azurin <jmazurin@us.ibm.com>
1 file changed