Configure Kotlin: add mavenCentral() to repositories

Since kotlin stdlib depends on artifacts from mavenCentral() we should add it
 #KT-18719 Fixed

idea/testData/configuration/gsk/eap11Version/build_after.gradle.kts

@@ -9,6 +9,7 @@ buildscript {
         maven {
             setUrl("http://dl.bintray.com/kotlin/kotlin-eap-1.1")
         }
+        mavenCentral()
     }
     dependencies {
         classpath(kotlinModule("gradle-plugin", kotlin_version))

idea/testData/configuration/gsk/eapVersion/build_after.gradle.kts

@@ -8,6 +8,7 @@ buildscript {
         maven {
             setUrl("http://dl.bintray.com/kotlin/kotlin-eap")
         }
+        mavenCentral()
     }
     dependencies {
         classpath(kotlinModule("gradle-plugin", kotlin_version))