bazel: bump rules_go version to 0.18.1
bazel: bump rules_go version to 0.18.1

We were stuck on an old version of rules_go so that we could use Go 1.9
as required by the old version of kubernetes. Now that we're on Go
1.10, bump rules_go to be the latest version available.

sipb/homeworld,sipb/homeworld,sipb/homeworld,sipb/homeworld