added clear functionality to Jruby::Mapdb::Tree

Signed-off-by: Christian MICHON <christian.michon@gmail.com>
release 0.9.6

This gem's revision scheme will not follow MapDB revision anymore:
- Else it would enforce to wait for new releases of MapDB to publish fixes.
- Revision used of MapDB will appear in README.md instead

Signed-off-by: Christian MICHON <christian.michon@gmail.com>