Change randomIP() function to only generate "valid" IPs.

See this part of BridgeDB's documentation [0] for how we determine if an IP
address is "valid".

[0]: https://para.noid.cat/bridgedb/bridgedb.parse.html#how-address-validity-is-determined
Merge branch 'release-1.1.4'
Merge branch 'master' into develop