Make use of IPv4 vs IPv6 explicit in Sentry's UDP listener

[crankycoder]

#988 uses IPv6 if it's available and falls back to IPv4 when listening on UDP.

It would be much nicer if Sentry defaulted to IPv4 which is still the normal use case for most people and the option to use IPv6 was made explicit in the configuration file.

[dcramer]

UDP is gone