Bump org.apache.maven.plugins:maven-checkstyle-plugin

Bumps [org.apache.maven.plugins:maven-checkstyle-plugin](https://github.com/apache/maven-checkstyle-plugin) from 3.3.0 to 3.3.1.
- [Commits](apache/maven-checkstyle-plugin@maven-checkstyle-plugin-3.3.0...maven-checkstyle-plugin-3.3.1)

---
updated-dependencies:
- dependency-name: org.apache.maven.plugins:maven-checkstyle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

pom.xml

@@ -23,7 +23,7 @@
         <maven.compiler.target>1.7</maven.compiler.target>
         <rxjava.version>1.3.8</rxjava.version>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-        <checkstyle.version>3.3.0</checkstyle.version>
+        <checkstyle.version>3.3.1</checkstyle.version>
         <findbugs.version>3.0.5</findbugs.version>
         <javadoc.version>3.6.0</javadoc.version>
         <pmd.version>3.21.0</pmd.version>