Make toxenv match run environments

[ryanhiebert]

Currently the toxenv after all option looks at the TOXENV environment variable. This is mostly useful, because it's quite common to set that environment variable to control tox, and tox-travis uses it under the hood.

However, it fails in a couple of cases:

1. No TOXENV is set. This is mostly a non-concern for us, because we always set it.
2. -e env is given to tox. This is the more annoying part, because it might be used. It's a reasonable limitation for a first version, but there's probably going to be someone somewhere that is surprised by this, and I won't blame them.

What I'd like to do, if possible, is inspect Tox's run somehow, so that I know which envs were actually run, and match against those instead of relying on the environment variable.