Bump maven to v3.1.1; add maven release script

Right now, you have to reverse-engineer the contents of http://s3.amazonaws.com/heroku-jvm-langpack-java/maven.tar.gz if you want to compile and release a new version of `maven` for your buildpack. This PR provides developers a way to package new releases of `maven` with this buildpack, as well as bump the default version of `maven` to v3.1.1.

I would like the tarballs to be retrieved from Heroku's S3 bucket as opposed to my swift container on HP Public Cloud (so as to not eat up costs on my end), so please let me know if you guys are interested in merging this, and I will change the endpoint URL back to http://s3.amazonaws.com/heroku-jvm-langpack-java/ :smile: