cassandra: Fix JNA link errors by downgrading to 3.2.7

Use JNA 3.2.7 which is what I was using with Cassandra earlier to
resolve link errors.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Merge pull request #3 from penberg/fix-cassandra-jna

cassandra: Fix JNA link errors by downgrading to 3.2.7