Clear timerFunctionForBufferedLogs if not needed

This is an example of a fix for loggly/winston-loggly-bulk#12 with a small amount of code change.

Note that these lines:
```
    if (timerFunction === null) {
      timerFunction = setInterval(function () {
        sendBulkLogs();
      },30000);
    }
```

produce the same bug when `isBulk` is `true`. See loggly/winston-loggly-bulk#13

lib/loggly/common.js

@@ -222,8 +222,11 @@ common.loggly = function () {
   //
   if (timerFunctionForBufferedLogs === null) {
     timerFunctionForBufferedLogs = setInterval(function () {
-      if (arrBufferedMsg.length) sendBufferdLogstoLoggly();
+      if (arrBufferedMsg.length) { sendBufferdLogstoLoggly(); }
     }, bufferOptions.retriesInMilliSeconds);
+  } else if (timerFunctionForBufferedLogs && !arrBufferedMsg.length) {
+    clearInterval(timerFunctionForBufferedLogs);
+    timerFunctionForBufferedLogs = null;
   }