Upgrade org.codehaus.plexus:plexus-component-metadata to 1.7.1
This will make them build with Java9

git-svn-id: https://svn.apache.org/repos/asf/maven/shared/trunk@1772107 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/pom.xml b/pom.xml
index 036e25a..87d7ef9 100644
--- a/pom.xml
+++ b/pom.xml
@@ -128,7 +128,7 @@
       <plugin>
         <groupId>org.codehaus.plexus</groupId>
         <artifactId>plexus-component-metadata</artifactId>
-        <version>1.6</version>
+        <version>1.7.1</version>
         <executions>
           <execution>
             <goals>