ERR define missed (#27)

diff --git a/lib/skywalking/client.lua b/lib/skywalking/client.lua
index 7312a26..0f0a292 100644
--- a/lib/skywalking/client.lua
+++ b/lib/skywalking/client.lua
@@ -29,6 +29,7 @@
 
     local log = ngx.log
     local DEBUG = ngx.DEBUG
+    local ERR = ngx.ERR    
 
     check = function(premature)
         if not premature then