Update Mockito to 2.3.9

Author: nhaarman

mockito-kotlin/build.gradle

@@ -28,7 +28,7 @@ repositories {
 dependencies {
     compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
     compile "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
-    compile "org.mockito:mockito-core:2.2.28"
+    compile "org.mockito:mockito-core:2.3.9"
 
     /* Tests */
     testCompile "junit:junit:4.12"