maven配置国内镜像，优化 jar 包下载速度

maven 中央仓库在国外，下载不稳定，可以配置国内镜像

以阿里云的镜像为例：

修改 maven 的settings.xml，更新 <mirrors>标签内的内容：

<mirrors>
  <!-- mirror
    Specifies a repository mirror site to use instead of a given repository. The repository that
    this mirror serves has an ID that matches the mirrorOf element of this mirror. IDs are used
    for inheritance and direct lookup purposes, and must be unique across the set of mirrors.
  <mirror>
    <id>mirrorId</id>
    <mirrorOf>repositoryId</mirrorOf>
    <name>Human Readable Name for this Mirror.</name>
    <url>http://my.repository.com/repo/path</url>
  </mirror>
   -->

  <mirror>
    <id>alimaven</id>
    <name>aliyun maven</name>
    <url>http://maven.aliyun.com/nexus/content/groups/public/</url>
    <mirrorOf>central</mirrorOf>
  </mirror>

  <mirror>
    <id>uk</id>  
    <mirrorOf>central</mirrorOf>  
    <name>Human Readable Name for this Mirror.</name>  
    <url>http://uk.maven.org/maven2/</url>
  </mirror>

   <mirror>
    <id>CN</id>
    <name>OSChina Central</name>
    <url>http://maven.oschina.net/content/groups/public/</url>
    <mirrorOf>central</mirrorOf>
  </mirror>

  <mirror>
    <id>nexus</id>
    <name>internal nexus repository</name>
    <!-- <url>http://192.168.1.100:8081/nexus/content/groups/public/</url>-->
    <url>http://repo.maven.apache.org/maven2</url>
    <mirrorOf>central</mirrorOf>
  </mirror>

</mirrors>