Fix for static file saving

There is a line where the thumbnailer creates the directory where the static thumbnail file is saved for later use: [ThumbnailResponder.php#L180](https://github.com/bolt/bolt-thumbs/blob/master/src/ThumbnailResponder.php#L180)

According to [PHP manual](http://php.net/manual/en/function.mkdir.php#refsect1-function.mkdir-errors) we need an extra check if the directory exist, otherwise mkdir throws an E_WARNING level error which translated to Excpetion by Bolt and the file will not be saved at the next line because the try catch block.