Unfortunately there is not a stable release for the 1.12 version of Spigot yet! Some of our plugins have broken with the update so I can't in good faith move forward until they update. (Specifically Worldguard, no protections - eep!)
AS SOON as our server host releases 1.12 I will switch the server over but for right now you're going to have to stick with 1.11.2.
Thanks for your patience, and look forward to some cool stuff coming to the server in 1.12! (Including public release.) :)