Gradle: Add plug-in net.ltgt.apt-idea. (census-instrumentation#1523)

Author: songy23

build.gradle

@@ -37,6 +37,7 @@ subprojects {
     apply plugin: 'ru.vyarus.animalsniffer'
     apply plugin: 'findbugs'
     apply plugin: 'net.ltgt.apt'
+    apply plugin: 'net.ltgt.apt-idea'
     apply plugin: "me.champeau.gradle.jmh"
     apply plugin: "io.morethan.jmhreport"
     // Plugins that require java8