bump gradle wrapper to 1.9. Travis build breaks with gradle 1.8 & JDK 1....

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.