golang package should recommend golang-race-detector runtime

Bug 1487010 (original description "go1.5rc1: go test -race failing when building test exec on wily") was fixed by uploading a new package. but people won't get the fix unless they are very determined. Making the golang-go package recommend the race detector runtime when available will restore the behaviour from vivid and earlier, where the race detector just works once golang-go is installed. (It could be a Depends I guess, but that seems over the top as the go package is mostly usable without it).