Change Kotlin jvmTarget back to 1.8

It looks like Kotlin support in the IntelliJ IDEA
doesn't not carry the `jvmTarget = '11'` properly
and it still stick with `1.6` in the code editor

See more info in the SO: https://stackoverflow.com/questions/52888341/does-kotlin-support-java-11

build.gradle

@@ -210,7 +210,7 @@ configure(javaProjects) { subproject ->
 	}
 	compileTestKotlin {
 		kotlinOptions {
-			jvmTarget = '11'
+			jvmTarget = '1.8'
 		}
 	}