Gradle: do not add ext prefix for kotlin_version property

#KT-4690 Fixed

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

@@ -20,7 +20,7 @@ android {
 
 dependencies {
     compile 'com.android.support:support-v4:13.0.0'
-    compile "org.jetbrains.kotlin:kotlin-stdlib:$ext.kotlin_version"
+    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
 }
 buildscript {
     ext.kotlin_version = '$VERSION$'
@@ -28,6 +28,6 @@ buildscript {
         mavenCentral()
     }
     dependencies {
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$ext.kotlin_version"
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
     }
 }