tox: remove "skip_install", causes problems with latest tox-2.9.0
tox: remove "skip_install", causes problems with latest tox-2.9.0

We have "usedevelop = True". Previously, I think this didn't work *unless* we
also had "skip_install = True", but tox-2.9.0 fixed this bug, and now
skip_install overrides usedevelop, so having both of them prevents
Foolscap (or Twisted's "trial" executable) from being installed at all.

This *might* cause problems on older versions of Tox.. I've done a cursory
test against tox-2.7.0, which seemed ok, but, I'm not sure. I'd like to avoid
bumping the "minversion=" if at all possible, since that makes the "how to
test" instructions more complicated for environments that include
some (older) version of tox already. But if you see problems, please file a
bug and I'll bump it.

warner/foolscap