Upgrade Mockito 2.18.3 -> 2.21.0 (#31)

See:
- https://github.com/mockito/mockito/blob/release/2.x/doc/release-notes/official.md
- mockito/mockito@v2.18.3...v2.21.0

pom.xml

@@ -171,7 +171,7 @@
             <dependency>
                 <groupId>org.mockito</groupId>
                 <artifactId>mockito-core</artifactId>
-                <version>2.18.3</version>
+                <version>2.21.0</version>
             </dependency>
             <dependency>
                 <groupId>org.reactivestreams</groupId>