Upgrade to maven-checkstyle-plugin 2.16

This also upgrades Checkstyle to 6.2.  Release notes:

https://mail-archives.apache.org/mod_mbox/maven-announce/201507.mbox/%3C12606732.01437413806439.JavaMail.Dennis%40Dennis-Dator%3E
http://checkstyle.sourceforge.net/releasenotes.html

pom.xml

@@ -74,7 +74,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-checkstyle-plugin</artifactId>
-        <version>2.15</version>
+        <version>2.16</version>
         <executions>
           <execution>
             <id>check</id>