Upgrade to mockito-core 2.12.0

Fixes gh-1125

Author: kazuki43zoo

pom.xml

@@ -217,7 +217,7 @@
     <dependency>
       <groupId>org.mockito</groupId>
       <artifactId>mockito-core</artifactId>
-      <version>2.8.47</version>
+      <version>2.12.0</version>
       <scope>test</scope>
     </dependency>
     <!-- Do not go to 2.x until we are on jdk7 -->