Bump mockito-core from 2.20.1 to 2.21.0 (#43)

Bumps [mockito-core](https://github.com/mockito/mockito) from 2.20.1 to 2.21.0.
- [Release notes](https://github.com/mockito/mockito/releases)
- [Commits](mockito/mockito@v2.20.1...v2.21.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

pom.xml

@@ -44,7 +44,7 @@
         <maven.surefire.plugin.version>2.22.0</maven.surefire.plugin.version>
         <maven.javadoc.plugin.version>3.0.1</maven.javadoc.plugin.version>
         <plexus-component-metadata.version>1.7.1</plexus-component-metadata.version>
-        <mockito.version>2.20.1</mockito.version>
+        <mockito.version>2.21.0</mockito.version>
         <junit.version>4.12</junit.version>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     </properties>