Fix Spigot compatibility for build 1628 and higher

Since build 1628, Spigot checks the field named "protocol_version". It caused a NPE because NPCFactory doesn't set this field.

This commit fixes this and is compatible with normal Bukkit.

The value of this field might cause problems in the future but, for now, the value is not important.
Merge pull request #15 from MinePod/1.8-protocol-compatibility

1.8 protocol compatibility