Travis CI: run coverage test using --fail-env-changed (#3541)

diff --git a/.travis.yml b/.travis.yml
index 148c0eb..2e0ad87 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -56,7 +56,7 @@
             ./venv/bin/python -m test.pythoninfo
       script:
         # Skip tests that re-run the entire test suite.
-        - ./venv/bin/python -m coverage run --pylib -m test -uall,-cpu -x test_multiprocessing_fork -x test_multiprocessing_forkserver -x test_multiprocessing_spawn
+        - ./venv/bin/python -m coverage run --pylib -m test --fail-env-changed -uall,-cpu -x test_multiprocessing_fork -x test_multiprocessing_forkserver -x test_multiprocessing_spawn
       after_script:  # Probably should be after_success once test suite updated to run under coverage.py.
         # Make the `coverage` command available to Codecov w/ a version of Python that can parse all source files.
         - source ./venv/bin/activate