Travis: do not cache tox dirs (#564)

Re-using an existing `.tox` dir will result in less informative error
messages, e.g. when `python_requires` is broken [1].

1: https://travis-ci.org/pytest-dev/pytest-django/jobs/310792370

Author: blueyed

.travis.yml

@@ -1,4 +1,3 @@
-# Use container-based environment (faster startup, allows caches).
 sudo: false
 language: python
 dist: trusty
@@ -39,10 +38,6 @@ matrix:
   allow_failures:
     - env: TOXENV=py36-djmaster-postgres
 
-cache:
-  directories:
-    - "${TRAVIS_BUILD_DIR}/.tox/${TOXENV}"
-
 install:
   # Create pip wrapper script, using travis_retry (a function) and
   # inject it into tox.ini.
@@ -83,6 +78,3 @@ after_success:
       codecov --required -X search gcov pycov -f coverage.xml --flags $codecov_flags
     fi
     set +x
-
-before_cache:
-  - "find ${TRAVIS_BUILD_DIR}/.tox/${TOXENV} -name 'log' -o -name '__pycache__' -type d | xargs -I {} rm -rf {}"