Actually test against multiple Kotlin versions

Author: nhaarman

tests/build.gradle

@@ -1,6 +1,5 @@
 buildscript {
-    ext.kotlin_version = System.getenv("KOTLIN_VERSION") ?: '1.0.7'
-    ext.kotlin_version = '1.3.0'
+    ext.kotlin_version = System.getenv("KOTLIN_VERSION") ?: '1.3.0'
 
     repositories {
         mavenCentral()