Updating server dependancies. (#7538)

vendor/github.com/mattermost/html2text/LICENSE

@@ -1,6 +1,7 @@
 The MIT License (MIT)
 
 Copyright (c) 2015 Jay Taylor
+Modified work: Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal