Trim the build for Play! 2 applications

Drop the following from the slug

* `${BUILD_DIR}/project/boot`
* `${BUILD_DIR}/.jdk`
* `${BUILD_DIR}/target/scala-*`
* `${BUILD_DIR}/target/streams`
* `${BUILD_DIR}/target/resolution-cache`

This reduced my slug size from 142MB to 39MB. By comparison, `sbt dist` creates a 34MB zip file.