The site where gradle pulls the LunatriusCore dependency from is down, preventing any development environment setup/compiling.
I solved this by using jitpack:

repositories {
	maven { url 'https://www.jitpack.io' }
}

dependencies {
	deobfCompile group: 'com.github.Lunatrius', name: 'LunatriusCore', version: 'master-SNAPSHOT', classifier: 'universal'
}

Yeah this is very much a temporary solution.

My website is currently down. That might work as a temporary work around but it's not a permanent solution.