README: maven added to Fedora prerequisites

Build failed on my Fedora 20 due to lack of maven package.

Reviewed-by Zhi Yong Wu <zwu.kernel@gmail.com>
Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>