write_riemann: Use riemann-c-client 1.6.0+

write_riemann: Use riemann-c-client 1.6.0+

Use a newer version of riemann-c-client, so that we can use
riemann_message_get_packed_size(), which was introduced partly to support
collectd's use of the library.

Also use riemann_event_string_attribute_add(), introduced in the same version,
to make a lot of the code simpler.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>