build: travis download gradle

.travis.yml

@@ -1,10 +1,15 @@
 sudo: false
 language: java
 env:
- - GRADLE_USER_HOME=gradle-cache
+  - GRADLE_USER_HOME=gradle-cache
 cache:
   directories:
-   - gradle-cache
-   - netbeans
+    - gradle-cache
+    - netbeans
 jdk:
   - oraclejdk8
+before_install:
+  - wget http://services.gradle.org/distributions/gradle-2.13-bin.zip
+  - unzip gradle-2.13-bin.zip
+  - export GRADLE_HOME=$PWD/gradle-2.13
+  - export PATH=$GRADLE_HOME/bin:$PATH