Make riak keep data and logs under /usr/local/var subfolders

This change alters default riak configuration, so it will keep data in /usr/local/var/lib/riak and logs in /usr/local/var/log/riak. With this setup, your data will survive when installing new version of riak.

I also managed to simplify whole cellar structure, just by changing RIAK_BASE_DIR in env.sh

Don't use `unless...else`, either use `if build.devel?` and switch the order of the branches or use `if build.stable?` here.