Update checkstyle plugin version

- https://jira.codehaus.org/browse/MCHECKSTYLE-230

Change-Id: I46940052e5904e562317c12bd0f2185e46be7eb2
diff --git a/pom.xml b/pom.xml
index 00fc37a..ea879ea 100644
--- a/pom.xml
+++ b/pom.xml
@@ -431,7 +431,7 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-checkstyle-plugin</artifactId>
-                <version>2.12.1</version>
+                <version>2.13</version>
                 <dependencies>
                     <dependency>
                         <groupId>org.onlab.tools</groupId>