Enable ES groovy sandbox

.travis.yml

@@ -21,6 +21,7 @@ before_script:
   - mkdir ${HOME}/elasticsearch
   - wget https://download.elastic.co/elasticsearch/elasticsearch/elasticsearch-${ES_VERSION}.tar.gz
   - tar -xzf elasticsearch-${ES_VERSION}.tar.gz -C ${HOME}/elasticsearch
+  - echo 'script.groovy.sandbox.enabled: true' >> ${HOME}/elasticsearch/elasticsearch-${ES_VERSION}/config/elasticsearch.yml
   - ${HOME}/elasticsearch/elasticsearch-${ES_VERSION}/bin/elasticsearch >/dev/null &
 
 install: