Update README to show 2.x artifact badge

Author: nhaarman

README.md

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 # Mockito-Kotlin
-[ ![Download](https://maven-badges.herokuapp.com/maven-central/com.nhaarman/mockito-kotlin/badge.svg) ](https://maven-badges.herokuapp.com/maven-central/com.nhaarman/mockito-kotlin)
+[ ![Download](https://maven-badges.herokuapp.com/maven-central/com.nhaarman.mockitokotlin2/mockito-kotlin/badge.svg) ](https://maven-badges.herokuapp.com/maven-central/com.nhaarman.mockitokotlin2/mockito-kotlin)
 
 A small library that provides helper functions to work with [Mockito](https://github.com/mockito/mockito) in Kotlin.