Upgrade to mockito 2.23.0

Fixes gh-327

Author: kazuki43zoo

pom.xml

@@ -229,7 +229,7 @@
     <dependency>
       <groupId>org.mockito</groupId>
       <artifactId>mockito-core</artifactId>
-      <version>2.15.0</version>
+      <version>2.23.0</version>
       <scope>test</scope>
     </dependency>