travis: use tox, temporarily disable coverage

I don't yet know how to get tox to run the equivalent of "coverage run
`which trial` wormhole.test", since tox doesn't use a shell. But I want
to switch travis to tox so we get the same dependencies (in particular
the pyflakes constraint) everywhere. I'll figure out how to re-enable
coverage data soon.

.travis.yml

@@ -12,13 +12,6 @@ python:
   - "3.4"
   - "3.5"
 install:
-  - pip install -U pip
+  - pip install -U pip tox virtualenv
-  - pip install pyflakes coverage codecov
-  - pip install --editable .
 script:
-  - pyflakes setup.py src
+  - tox -e py
-  - wormhole --version
-  - coverage run --branch `which trial` wormhole
-after_success:
-  - coverage report
-  - codecov