allow failures in es master

due to elasticsearch master containing breaking changes as it moves to 7.0.0,
it's not required that the tests pass for this particular scenario until
it reaches closer to 7.0.0-alpha1

Author: jsvd

.travis.yml

@@ -23,6 +23,8 @@ matrix:
     env: LOGSTASH_BRANCH=6.0
   - rvm: jruby-1.7.27
     env: LOGSTASH_BRANCH=5.6
+  allow_failures:
+  - env: INTEGRATION=true ES_VERSION=master TEST_DEBUG=true
   fast_finish: true
 install: true
 script: ci/build.sh