The Java Buildpack automatically chooses a JRE for you based on it's current configuration and a preference for staying secure. So at the moment,
it's configured with
1.8.0_+ (a wildcard version to stay on the latest 1.8) and that resolves to
1.8.0_25 today. I'd recommend using the buildpack in its default form unless you've tested your application with it and experience problems. If you do need to change the version,
fork and update the configuration.