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"))

plugins/annotation-based-compiler-plugins-ide-support/build.gradle.kts

@@ -1,6 +1,8 @@
 
 apply { plugin("kotlin") }
 
+jvmTarget = "1.6"
+
 dependencies {
     compile(project(":compiler:util"))
     compile(project(":compiler:frontend"))

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

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

plugins/sam-with-receiver/sam-with-receiver-ide/build.gradle.kts

@@ -3,6 +3,8 @@ description = "Kotlin SamWithReceiver IDEA Plugin"
 
 apply { plugin("kotlin") }
 
+jvmTarget = "1.6"
+
 dependencies {
     compile(project(":kotlin-sam-with-receiver-compiler-plugin"))
     compile(project(":plugins:annotation-based-compiler-plugins-ide-support"))