update nexus-staging-maven-plugin to latest version

Author: stefanseifert

pom.xml

@@ -356,7 +356,7 @@ under the License.
         <plugin>
           <groupId>org.sonatype.plugins</groupId>
           <artifactId>nexus-staging-maven-plugin</artifactId>
-          <version>1.6.6</version>
+          <version>1.6.8</version>
         </plugin>
         <plugin>
           <groupId>external.atlassian.jgitflow</groupId>