Use default GOPATH, when env variable is empty

[bfanger]

When the GOPATH is empty (which is optional since go 1.8) the watcher panics

I've patched watcher.generateBinaryPrefix() to use:

gopath := os.Getenv("GOPATH")
if gopath == "" {
    gopath = build.Default.GOPATH
}

I also fixed some linter issues, so go test passes again.