Release 1.6.0: add compatibility with HeadsPluginAPI 2.x
This is the release that makes the plugin compatible with HeadsPluginAPI 2.x, and therefore also with minecraft 1.13.0 - 1.16.x
NOTE: if you are running a 1.16.1 mc server, make sure that you have HeadsPluginAPI 2.0.4 or newer! There is a bug in older versions of HeadsPluginAPI related to changes in the format of NBT tags on head blocks in 1.16.1.