Configure Kotlin: add mavenCentral() to repositories

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

idea/testData/configuration/android-gradle/missedRepositoriesInBuildscriptBlock_after.gradle

@@ -8,6 +8,7 @@ buildscript {
         maven {
             url 'http://oss.sonatype.org/content/repositories/snapshots'
         }
+        mavenCentral()
     }
 }
 apply plugin: 'android'
@@ -35,4 +36,4 @@ dependencies {
     compile "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
 }
 
-// VERSION: $VERSION$
+// VERSION: $VERSION$