Module isStream should be isstream

Author: mneil

lib/winston/transports/file.js

@@ -13,7 +13,7 @@ var events = require('events'),
     colors = require('colors'),
     common = require('../common'),
     Transport = require('./transport').Transport,
-    isWritable = require('isStream').isWritable,
+    isWritable = require('isstream').isWritable,
     Stream = require('stream').Stream;
 
 //