Don't use Tox in Travis

[stasm]

According to https://docs.travis-ci.com/user/languages/python/#travis-ci-uses-isolated-virtualenvs, using both Travis and Tox isn't well supported. Travis already runs all tests from the matrix in separate virtualenvs. Can we drop Tox?

[Pike]

I like testing in tox, because that's what works locally. Like, I don't see how you can test fluent-runtime locally w/out installing both fluent.syntax and fluent.runtime into a virtualenv.

[spookylukey]

We could use tox for local testing and just make Travis do something different. We don't have a particularly complex setup, so the bit of duplication involved shouldn't be too hard to manage.

[stasm]

Alright, this makes sense. Thank you to both of you for your input. Let's add Python 3.7 to Travis in #70 and then, let's try to drop Tox from .travis.yml in this issue. We'll leave Tox for local testing.

[Pike]

I'm closing this, 'cause we're not using tox in travis.

Not that our .travis.yml is in good shape, but that's a different bug.