bump checkstyle version to build on Java 9 (googleapis#3577)

pongad · web-flow · commit 85645a6641ba · 2018-08-20T09:55:00.000-07:00
The version of checkstyle we currently use uses tools.jar
which is removed from Java 9 and above as part of Project Jigsaw.
This commit uses a newer version of checkstyle that does not
use tools.jar.

Running `mvn checkstyle:checkstyle` succeeded.
diff --git a/google-cloud-clients/pom.xml b/google-cloud-clients/pom.xml
@@ -660,7 +660,7 @@
           <dependency>
             <groupId>com.puppycrawl.tools</groupId>
             <artifactId>checkstyle</artifactId>
-            <version>6.15</version>
+            <version>6.16</version>
           </dependency>
         </dependencies>
         <executions>
