Gradle: Compile Gradle models/model builders for plugins under JVM 1.6

This prevents imports failure in projects using older versions of Gradle

plugins/allopen/allopen-ide/build.gradle.kts

@@ -3,6 +3,8 @@ description = "Kotlin AllOpen IDEA Plugin"
 
 apply { plugin("kotlin") }
 
+jvmTarget = "1.6"
+
 dependencies {
     compile(project(":kotlin-allopen-compiler-plugin"))
     compile(project(":compiler:util"))