import gopacket from github instaead of google code

The latter redirects to the former, which breaks `go get`.

Closes #1071.

router/ethernet_decoder.go

@@ -2,8 +2,8 @@ package router
 
 import (
 	"bytes"
-	"code.google.com/p/gopacket"
-	"code.google.com/p/gopacket/layers"
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
 	"log"
 	"net"
 )