Add <version> to gpg plugin in release profile as well

Without this the build uses plugin version 1.1 during a release. This
should fix the problems we had on Jenkins.

Author: bernd

pom.xml

@@ -268,6 +268,7 @@
                     <plugin>
                         <groupId>org.apache.maven.plugins</groupId>
                         <artifactId>maven-gpg-plugin</artifactId>
+                        <version>1.6</version>
                         <configuration>
                             <keyname>B1606F22</keyname>
                         </configuration>