Use ipv4 address by default for service instances (#147)
Use ipv4 address by default for service instances (#147)

* -- use ipv4 address by default for service instances created by discovery

* -- feedback from @dposada
 - throw exception if no IPv4 address is available

twosigma/waiter,twosigma/waiter,twosigma/waiter,twosigma/waiter