Update gradle wrapper version to 1.9. Should fix Travis CI build which is using the 1.8 wrapper. 

I reproduced the current travis build issue at https://travis-ci.org/eriwen/gradle-js-plugin
The gradle-js-plugin build fails with JDK 1.7 and gradle 1.8. Bumping to 1.9 seems to fix the issue. Hopefully this change will allow the travis build to download the correct version of gradle.