Upgrade Java Buildpack

This change updates the submodule for the Java Buildpack from
https://github.com/cloudfoundry/cloudfoundry-buildpack-java to
https://github.com/cloudfoundry/java-buildpack.  There are many
improvements in the new buildpack, and its README.md should be
referred to directly for more details.

[#52324703]
Merge pull request #49 from nebhale/52324703-curated-buildpack

Upgrade Java Buildpack

@jfoley / @ryantang