Stop trying to detect Go version

It fails on pre-release versions, anyway.  It's enough to mention the oldest
supported version in README.md.  Anything older than two latest Go releases
is bad idea, anyway, 'cause it's unsupported by Go development team.

Closes #680

build.go

@@ -264,24 +264,14 @@ type Constants map[string]string
 func (cs Constants) LDFlags() string {
 	l := make([]string, 0, len(cs))
 
-	if runtime.Version() < "go1.5" {
-		for k, v := range cs {
-			l = append(l, fmt.Sprintf(`-X %q %q`, k, v))
-		}
-	} else {
-		for k, v := range cs {
-			l = append(l, fmt.Sprintf(`-X "%s=%s"`, k, v))
-		}
+	for k, v := range cs {
+		l = append(l, fmt.Sprintf(`-X "%s=%s"`, k, v))
 	}
 
 	return strings.Join(l, " ")
 }
 
 func main() {
-	if runtime.Version() < "go1.6" {
-		fmt.Fprintf(os.Stderr, "old version of Go detected (%v), I'll try but no guarantees\n", runtime.Version())
-	}
-
 	buildTags := []string{}
 
 	skipNext := false