Client build now requires go 1.13

The README says go 1.9+ is required, but a few recent commits have introduced `strings.ReplaceAll` (which needs go 1.12) and `errors.As` (which needs go 1.13) to the codebase. Either that was a mistake, or the README needs to be updated.

Start with the Go version requirement in README and verify the minimum versions required by the strings.ReplaceAll and errors.As usages mentioned in the issue. Done means the documented requirement matches the codebase, or the newer usages are addressed if they were introduced by mistake.

Written by the indexing model from the issue text.