Configure gpg plugin version in pluginManagement.

Maven is not using the 1.6 version of the GPG plugin for some reason, even
though the place where the plugin is triggered explicitly specified that
version. Maybe this new set of chicken sacrifices will cause Maven to do the
right thing.

pom.xml

@@ -107,6 +107,12 @@
           <artifactId>maven-resources-plugin</artifactId>
           <version>3.0.2</version>
         </plugin>
+
+        <plugin>
+          <groupId>org.apache.maven.plugins</groupId>
+          <artifactId>maven-gpg-plugin</artifactId>
+          <version>1.6</version>
+        </plugin>
       </plugins>
     </pluginManagement>
   </build>
@@ -163,7 +169,6 @@
           <plugin>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-gpg-plugin</artifactId>
-            <version>1.6</version>
             <executions>
               <execution>
                 <id>sign-artifacts</id>