sbt should use java on PATH

The [`sbt` script](https://github.com/heroku/heroku-buildpack-scala/blob/447a0e99aedbd21853f49832cb995d182a159a1e/bin/compile#L104-L106) generated by the buildpack for use in run processes should use `java` on the `PATH` instead of `/usr/bin/java`; otherwise, the Java version set in `system.properties` is not respected.