Upgrade sortpom to latest version, make verification strict

Author: aaime

geowebcache/pom.xml

@@ -652,7 +652,7 @@
       <plugin>
         <groupId>com.github.ekryd.sortpom</groupId>
         <artifactId>sortpom-maven-plugin</artifactId>
-        <version>2.11.0</version>
+        <version>2.15.0</version>
         <executions>
           <execution>
             <phase>verify</phase>
@@ -666,6 +666,8 @@
           <keepBlankLines>true</keepBlankLines>
           <spaceBeforeCloseEmptyElement>false</spaceBeforeCloseEmptyElement>
           <createBackupFile>false</createBackupFile>
+          <verifyFail>stop</verifyFail>
+          <verifyFailOn>strict</verifyFailOn>
         </configuration>
       </plugin>