fix zk version

.travis.yml

@@ -21,7 +21,7 @@ env:
    - DISCOVERY_NODES=127.0.0.1:7171
    - HTTP_PERF=tcp://0.0.0.0:0
    - DOCKER_COMPOSE_VERSION=1.24.1
-   - ZK_VERSION=3.5.5
+   - ZK_VERSION=3.5.6
 
 before_install:
   # docker-compose
@@ -30,7 +30,7 @@ before_install:
   - chmod +x docker-compose
   - sudo mv docker-compose /usr/local/bin
   # zookeeper
-  - wget "http://apache.cs.utah.edu/zookeeper/zookeeper-${ZK_VERSION}/zookeeper-3.5.4-beta.tar.gz"
+  - wget "http://apache.cs.utah.edu/zookeeper/zookeeper-${ZK_VERSION}/zookeeper-${ZK_VERSION}.tar.gz"
   - tar -xvf "zookeeper-${ZK_VERSION}.tar.gz"
 
 # Skip the install step. Don't `go get` dependencies. Only build with the code