shairport: fix buffer config var

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
shairport: update to latest version, add license and switch release to source version

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Merge pull request #233 from Noltari/shairport-next

Shairport: update, add license, config fix