Fix file size limit for media_upload

According to https://dev.twitter.com/rest/reference/post/media/upload
the limit of this endpoint is 5MB (which is 4883KiB) per file.

tweepy/api.py

@@ -198,7 +198,7 @@ class API(object):
             :allowed_param:
         """
         f = kwargs.pop('file', None)
-        headers, post_data = API._pack_image(filename, 3072, form_field='media', f=f)
+        headers, post_data = API._pack_image(filename, 4883, form_field='media', f=f)
         kwargs.update({'headers': headers, 'post_data': post_data})
 
         return bind_api(