maven aliyun 私有仓库服务配置文件。
maven.aliyun.com代理了很多公共的maven仓库。使用maven.aliyun.com中的仓库地址作为下载源，速度更快更稳定，以下为仓库说明：
public https://maven.aliyun.com/nexus/content/groups/public central仓和jcenter仓的聚合仓
google https://maven.aliyun.com/nexus/content/repositories/google
gradle-plugin https://maven.aliyun.com/nexus/content/repositories/gradle-plugin
spring https://maven.aliyun.com/nexus/content/repositories/spring
spring-plugin https://maven.aliyun.com/nexus/content/repositories/spring-plugin
grails-core https://maven.aliyun.com/nexus/content/repositories/grails-core
apache snapshots https://maven.aliyun.com/nexus/content/repositories/apache-snapshots
