add go version prerequisite and debug warning (#1394)

* add go version prerequisite and debug warning

* merge duplicate content

* remove duplicate content

debug.go

@@ -47,6 +47,9 @@ func debugPrint(format string, values ...interface{}) {
 }
 
 func debugPrintWARNINGDefault() {
+	debugPrint(`[WARNING] Now Gin requires Go 1.6 or later and Go 1.7 will be required soon.
+
+`)
 	debugPrint(`[WARNING] Creating an Engine instance with the Logger and Recovery middleware already attached.
 
 `)