Upgrade to pynmsg revision 0.4.0.

pynmsg (0.4.0)

• Add missing 'nogil' annotations to nmsg_input_open_json(), nmsg_message_from_json()
• setup.py: Require libnmsg >= 0.10.0 due to JSON I/O
• Add JSON IO support
• setup.py: Use pkg-config to retrieve -I, -L, -l flags for libnmsg
• Add embedsignature preprocessor directive to all pyx files

Sponsored by: Farsight Security, Inc.