use constants for source/target compatibility levels

build.gradle

@@ -2,8 +2,8 @@ plugins {
 	id 'fabric-loom' version '0.2.0-SNAPSHOT'
 }
 
-sourceCompatibility = 1.8
+sourceCompatibility = JavaVersion.VERSION_1_8
-targetCompatibility = 1.8
+targetCompatibility = JavaVersion.VERSION_1_8
 
 archivesBaseName = "modid"
 version = "1.0.0"