Gradle: do not add ext prefix for kotlin_version property

#KT-4690 Fixed

idea/testData/configuration/android-gradle/gradleExamples/gradleExample18_after.gradle

@@ -5,7 +5,7 @@ buildscript {
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:0.5.+'
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$ext.kotlin_version"
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
     }
 }
 apply plugin: 'android'
@@ -20,7 +20,7 @@ dependencies {
     debugCompile 'com.android.support:support-v13:13.0.0'
 
     compile 'com.google.android.gms:play-services:3.1.36'
-    compile "org.jetbrains.kotlin:kotlin-stdlib:$ext.kotlin_version"
+    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
 }
 
 android {