Add gevent version to oldest-version tox environments

As a finding of that, gevent 0.13.0 does not work, so state the
reliance on 0.13.1.

gevent of the 0.13.0 vintage seems to crash when performing 'import
gevent', so it seems unlikely even a single such user relying on
0.13.0 exists.  I doubt amending this requirement would materially
hurt anyone, and may have never worked anyway.