py-chrome tests are flaky due to an issue with chromedriver

on alert handling we are encountering this bug:
https://bugs.chromium.org/p/chromedriver/issues/detail?id=1500

seems to occur at random. We either need to ignore the chrome alert tests
or wrap every alert call handling the error. until the chromedriver bug is fixed

Author: lukeis

.travis.yml

@@ -41,6 +41,7 @@ matrix:
   fast_finish: true
   allow_failures:
     - env: TASK=rb TARGET=remote-firefox-test
+    - env: TASK=py TOXENV=py27-chrome
     - env: TASK=py TOXENV=py27-firefox
     - env: TASK=py TOXENV=py27-marionette
     - env: TASK=py TOXENV=py27-remote