make rebar output package_version to pkg.vars.config file

The first step towards an easier riak/riak_cs setup. 

The goal is to remove the following manual configuration step. 

In the `/etc/riak/app.config` file, you must specify the following, ``` {add_paths, ["/usr/lib/riak-cs/lib/riak_cs-1.4.5/ebin"]},```.

That path changes on every release, which complicates configuration management. 

With the version information in the pkg.vars.config file perhaps we can do a copy or symlink the directory 
`/usr/lib/riak-cs/lib/riak_cs-<version>/ebin` to `/usr/lib/riak-cs/lib/riak_cs-latest/ebin`.