Upgrade to maven-checkstyle-plugin to 2.15

This also upgrades Checkstyle to 6.1.1.  Release notes:

https://jira.codehaus.org/secure/ReleaseNote.jspa?projectId=11127&version=20762
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>2.14</version>
+        <version>2.15</version>
         <executions>
           <execution>
             <id>check</id>