Convert flush time interval into ns

Flush interval must be in nanoseconds in comparison but it comes
from config in seconds.

Change-Id: I3da948f39118301dcd02c526d6cb5eea92a5b9d0
diff --git a/heka/files/lua/common/accumulator.lua b/heka/files/lua/common/accumulator.lua
index 9b01a8f..ac86a0e 100644
--- a/heka/files/lua/common/accumulator.lua
+++ b/heka/files/lua/common/accumulator.lua
@@ -47,7 +47,7 @@
 -- ns: the current timestamp in nanosecond (optional)
 function Accumulator:flush(ns)
     local now = ns or time() * 1e9
-    if #self.buffer > self.flush_count or now - self.last_flush > self.flush_interval then
+    if #self.buffer > self.flush_count or now - self.last_flush > self.flush_interval * 1e9 then
         self.flush_cb(self.buffer)
         self.buffer = {}
         self.last_flush = now