Upgrade to maven-gpg-plugin 1.6

Release notes:

https://mail-archives.apache.org/mod_mbox/maven-announce/201501.mbox/%3C20150120035915.E8B8E17775%40minotaur.apache.org%3E

pom.xml

@@ -53,7 +53,7 @@
           <plugin>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-gpg-plugin</artifactId>
-            <version>1.5</version>
+            <version>1.6</version>
             <executions>
               <execution>
                 <id>sign-artifacts</id>