明确指定MAVEN插件版本，避免构建出现大量警告信息。

[isaacdong]

警告信息：

[WARNING] Some problems were encountered while building the effective model for com.alibaba:dubbo-common:jar:2.8.4
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-source-plugin is missing. @ com.alibaba:dubbo-parent:2.8.4, /projects/dubbox/pom.xml, line 480, column 12
[WARNING]
...
[WARNING] Some problems were encountered while building the effective model for com.alibaba:dubbo:jar:2.8.4
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-javadoc-plugin is missing. @ com.alibaba:dubbo:[unknown-version], /projects/dubbox/dubbo/pom.xml, line 336, column 21

maven版本：

Apache Maven 3.3.9 (bb52d8502b132ec0a5a3f4c09453c07478323dc5; 2015-11-11T00:41:47+08:00)
Maven home: /nemo/maven
Java version: 1.7.0_67, vendor: Oracle Corporation
Java home: /nemo/jdk1.7.0_67/jre
Default locale: zh_CN, platform encoding: UTF-8
OS name: "linux", version: "2.6.32-504.30.3.el6.x86_64", arch: "amd64", family: "unix"