Reduce boilerplate for applying kotlin-gradle-plugin.

performance/build.gradle

@@ -1,6 +1,9 @@
 buildscript {
+    ext.kotlin_version = '+'
+
+    apply from: "$rootDir/gradle/kotlinGradlePlugin.gradle"
+
     repositories {
-        mavenCentral()
         maven {
             url "https://dl.bintray.com/jetbrains/kotlin-native-dependencies"
         }
@@ -8,7 +11,6 @@ buildscript {
 
     dependencies {
         classpath "org.jetbrains.kotlin:kotlin-native-gradle-plugin:+"
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:+"
     }
 }