Upgrade to maven-checkstyle-plugin 3.0.0

This also upgrades Checkstyle to 6.19, the last JDK 7-compatible
version.  Release notes:

https://issues.apache.org/jira/secure/ReleaseNote.jspa?projectId=12317223&version=12334384
http://checkstyle.sourceforge.net/releasenotes.html#Release_6.19

pom.xml

@@ -74,7 +74,7 @@
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-checkstyle-plugin</artifactId>
-        <version>2.17</version>
+        <version>3.0.0</version>
         <executions>
           <execution>
             <id>check</id>