ci: alternative implementation of travis wait

As official travis_wait is broken for years, we go back to good old
bash tricks, which are reliable.

Author: ssbarnea

.travis.yml

@@ -36,7 +36,15 @@ install:
   - pip install tox
 
 script:
-  - travis_wait 30 tox
+  # Ping stdout every 9 minutes or Travis kills build,
+  # while travis_wait does not show the command output while processing.
+  # https://docs.travis-ci.com/user/common-build-problems/#build-times-out-because-no-output-was-received
+  # https://github.com/travis-ci/travis-ci/issues/4190#issuecomment-353342526
+  - |
+    while sleep 9m; do
+    echo "====[ $SECONDS seconds still running ]===="
+    done &
+  - tox
 
 before_cache:
   - rm -rf $HOME/.cache/pip/log