sikuli-script.jar in Sikuli IDE project not provide in any repository so you must use it like a 3rd party jar
see: "Mavan's central repository will not answer on any Sikuli stuff" https://answers.launchpad.net/sikuli/+question/185713
"Gradle: Make a 3rd party jar available to local gradle repository" Gradle: Make a 3rd party jar available to local gradle repository