client: add login_fail_exit params, default is true

if login_fail_exit is false, when frpc first login to server failed, it
    will continues relogin to server every 30 seconds.

.travis.yml

@@ -2,8 +2,8 @@ sudo: false
 language: go
 
 go:
-    - 1.7.5
-    - 1.8
+    - 1.7.x
+    - 1.8.x
 
 install:
     - make