all: fix bug when client shutdown and reconnect, server response already use

1. if client is offline, server will release all resources
2. use a graceful method to shutdown go net.Listeners
3. add closeFlag for Conn, so startHeartBeat func can exit correctly now

.travis.yml

@@ -1,11 +1,9 @@
-go_import_path: github.com/fatedier/frp
 sudo: false
 language: go
 
 go:
     - 1.4.2
     - 1.5.2 
-    - tip
 
 install:
     - make