adding virtual memory count for elastic search

docker-library/elasticsearch#111

.travis.yml

@@ -23,6 +23,7 @@ before_install:
     - curl -L https://github.com/docker/compose/releases/download/1.7.1/docker-compose-`uname -s`-`uname -m` > docker-compose
     - sudo chmod +x docker-compose
     - sudo mv docker-compose /usr/local/bin
+    - sudo sysctl -w vm.max_map_count=262144
 
 script:  
     - docker version