misc/dist: minimum target requirement is 10.6 for Darwin

     As we've dropped support for Mac OS X 10.5, I think the generated .pkg
     should reflect that decision.
     But this CL make it impossible to generate pkg on Mac OS X 10.6, at least
     for me.

R=adg, bradfitz
CC=golang-dev
https://golang.org/cl/5798051

misc/dist/bindist.go

@@ -203,7 +203,7 @@ func (b *Build) Do() error {
 			"--id", "com.googlecode.go",
 			"--title", "Go",
 			"--version", "1.0",
-			"--target", "10.5")
+			"--target", "10.6")
 		targs = append(targs, targ)
 	case "windows":
 		// Create ZIP file.