Gradle: do not add ext prefix for kotlin_version property

#KT-4690 Fixed

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

@@ -5,7 +5,7 @@ buildscript {
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:0.2'
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$ext.kotlin_version"
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
     }
 }
 
@@ -42,5 +42,5 @@ repositories {
     mavenCentral()
 }
 dependencies {
-    compile "org.jetbrains.kotlin:kotlin-stdlib:$ext.kotlin_version"
+    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
 }