get zookeeper stable version from its distribution web site

Run the following command at one line:

zkver=$(curl http://www.apache.org/dist/zookeeper/stable/
|grep -o 'zookeeper-[0-9].[0-9].[0-9][0-9].tar.gz'
|(head -n1 && tail -n1)|sed 's/.tar.gz//')

The results:

echo $zkver
zookeeper-3.4.10