Comply with latest BukkitDev policy regarding auto updaters.

Uses the new curse server mods API instead of the RSS feed. I switched the project slug to the project id which can be found here: https://api.curseforge.com/servermods/projects?search=buycraft

People will now have to add their own curse api key to the config.yml file in /plugins/updater/config.yml and can globally disable auto updaters there (at least the ones that use this class).