.travis.yml: use gradle instead of maven

Author: thypon

.travis.yml

@@ -1,14 +1,6 @@
-sudo: false
 language: java
-install:
-  - mvn install -Delasticsearch.version=$ES_VERSION -DskipTests=true
 script:
-  - mvn -Delasticsearch.version=$ES_VERSION -Dtests.security.manager=false test
-env:
-  - ES_VERSION=1.5.1
-  - ES_VERSION=1.5.2
-  - ES_VERSION=1.6.0
-  - ES_VERSION=1.7.0
+  - gradle plugin
 cache:
   directories:
   - $HOME/.m2