cassandra: fix upstream/current link not getting overriden

We need to pass -n flag in addittion to -f, otherwise existing target
directory will be dereferenced and the link will be created in that
directory.
Cassandra 2.1.0

Upgrade Cassandra to 2.1.0.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>