Update Mockito to run on Java 11

(cherry picked from commit dc3454f)

Author: acogoluegnes

pom.xml

@@ -62,7 +62,7 @@
     <commons-cli.version>1.1</commons-cli.version>
     <junit.version>4.12</junit.version>
     <awaitility.version>3.1.0</awaitility.version>
-    <mockito.version>2.16.0</mockito.version>
+    <mockito.version>2.21.0</mockito.version>
 
     <maven.javadoc.plugin.version>3.0.0</maven.javadoc.plugin.version>
     <maven.release.plugin.version>2.5.3</maven.release.plugin.version>