Currently the license of this project isn't specified anywhere in the repository but the dev Bukkit page mentions that it's in the Public Domain so I personally would treat the code as is.

To not create some confusion or even illegal code usage by simply specifying a different license I would like to open discussion here with everyone who also contributed to this in the past (@baileyherbert, @phemmer, @FrankHeijden) about which license should be specified in the repository and the source code and if you accept the quasi re-licensing of your contributions.

I personally would favour GPLv3 but I understand that some might not be in favour with some of its terms. Closer to the original "Public Domain" statement might be one of the common MIT licenses or even the Unlicense.