Bump license year (#8512)

ca627655 · Angel_Kitty · Marc Cornellà · d9e64344 · ca627655
Commit ca627655 authored Jan 01, 2020 by Angel_Kitty Committed by Marc Cornellà Jan 01, 2020
--- a/LICENSE.txt
+++ b/LICENSE.txt
 MIT License

-Copyright (c) 2009-2019 Robby Russell and contributors (https://github.com/ohmyzsh/ohmyzsh/contributors)
+Copyright (c) 2009-2020 Robby Russell and contributors (https://github.com/ohmyzsh/ohmyzsh/contributors)

 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal