Fix compiling REI plugin

plugin/rei/build.gradle

@@ -56,6 +56,10 @@ tasks.withType(JavaCompile) {
 	options.encoding = "UTF-8"
 }
 
+compileKotlin {
+	kotlinOptions.jvmTarget = JavaVersion.VERSION_1_8
+}
+
 java {
 	// Loom will automatically attach sourcesJar to a RemapSourcesJar task and to the "build" task
 	// if it is present.