'build.plugins.plugin.version' for com.pyx4me:proguard-maven-plugin is missing. @ line 42, column 12

‘build.plugins.plugin.version’ for com.pyx4me:proguard-maven-plugin is missing. @ line 42, column 12

执行Maven Install的时候提示如下警告信息：

[WARNING] 
[WARNING] Some problems were encountered while building the effective model for xunge-middleware:xunge-middleware:war:0.0.1-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for com.pyx4me:proguard-maven-plugin is missing. @ line 42, column 12
[WARNING] 
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING] 
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING] 

其实警告信息忽略不管也可以的，不过这里的警告是关于插件版本的，最好还是指定版本：

<plugin>
                <groupId>com.pyx4me</groupId>
                <artifactId>proguard-maven-plugin</artifactId>
                **<version>2.0.4</version>**
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>proguard</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <obfuscate>true</obfuscate>
                    <proguardInclude>${basedir}/proguard.conf</proguardInclude>
                    <!-- 添加依赖，这里你可以按你的需要修改 -->
                    <libs>
                        <lib>${java.home}/lib/rt.jar</lib>
                        <lib>lib/jsp-api.jar</lib>
                        <lib>lib/servlet-api.jar</lib>
                    </libs>
                    <addMavenDescriptor>false</addMavenDescriptor>
                </configuration>
                <dependencies> 
                    <!-- 使用4.8版本来混淆 -->
                    <dependency>
                        <groupId>net.sf.proguard</groupId>
                        <artifactId>proguard</artifactId>  
                        <version>4.8</version>
                        <scope>runtime</scope>
                    </dependency>
                </dependencies>
            </plugin>

添加了<version>2.0.4</version>警告就消失了。