Add gradle wrapper

[Gradle wrapper](http://www.gradle.org/docs/current/userguide/gradle_wrapper.html) allows auto-install of Gradle. This is supposed to ease provisioning and updating of gradle (including on Travis too).

The gradle version provisioned by our chef recipes is quite dated. There is an [alternative cookbook](https://github.com/michaelklishin/gradle-chef-cookbook) which is up to date but I don't know if it's worth to bother with it when we have *Gradle wrapper*.

- [x] Add gradle wrapper
- [x] Check behaviour of Travis-CI builds
- [x] Update related contribution readme
- [x] Update test_vm cookbooks