Update flatten-maven-plugin to generate a flatten reduced POM

[lujiajing1126]

Since Arthas >=4.0.4, the generated POM cannot be resolved by Maven if it is imported in another project,

        <dependency>
            <groupId>com.taobao.arthas</groupId>
            <artifactId>arthas-core</artifactId>
            <version>4.0.4</version>
        </dependency>

The following error will be reported,

[ERROR] Failed to execute goal on project xxxxxxx: Could not resolve dependencies for project xxxxxxx: Failed to collect dependencies at com.taobao.arthas:arthas-core:jar:4.0.4: Failed to read artifact descriptor for com.taobao.arthas:arthas-core:jar:4.0.4: The following artifacts could not be resolved: com.taobao.arthas:arthas-all:pom:${revision} (absent): com.taobao.arthas:arthas-all:pom:${revision} was not found in https://maven.aliyun.com/repository/public during a previous attempt. This failure was cached in the local repository and resolution is not reattempted until the update interval of aliyun has elapsed or updates are forced -> [Help 1]

This issue may be introduced in commit 797baf5, also see this discussion mojohaus/flatten-maven-plugin#100

[lujiajing1126]

After upgrading the plugin,

The generated reduced pom is correctly flattened.