use precise distribution instead of trusty (#146)

Author: wangjoshuah

.travis.yml

@@ -26,4 +26,5 @@ before_script:
 script:
   - ./gradlew cleanAllModules
   - ./gradlew testAllModulesTravis
-  - if [[ -n $TRAVIS_TAG ]]; then ./gradlew ship; fi
+  - if [[ -n $TRAVIS_TAG ]]; then ./gradlew ship; fi
+dist: precise