Set the version of the jarjar-maven-plugin in pluginManagement (googleapis#515)

Author: chingor13

pom.xml

@@ -297,7 +297,11 @@
             <target>1.6</target>
           </configuration>
         </plugin>
-
+        <plugin>
+          <groupId>org.sonatype.plugins</groupId>
+          <artifactId>jarjar-maven-plugin</artifactId>
+          <version>1.9</version>
+        </plugin>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-source-plugin</artifactId>