package/mono: fixing mono version

This patch fixes to the correct mono version 4.0.0 as stated on
the download page.

Fixes:
http://autobuild.buildroot.net/results/227/227f942107e4fcbc14bb1e7d97e1e2758d6f3f2d/



[Peter: add autobuilder reference]
Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>