restore Version.getRevision() to be backward compatible

We removed the getRevision() function in 3.5.6:
https://github.com/apache/zookeeper/commit/96880c0b2ba39f1841f8bdc4a0119a467b462d03#diff-1d42dde48d42e3baf819e22b56a880a7

Some projects (like HBase for example) are actually using this method. (Although not relevant anymore, since we are on git).

3.5.6, 3.5.7 and 3.6.0 does not have this method anymore. However, if someone is using this function, to make upgrading easier, we should restore it.