Ivy依赖冲突时如何让低版本的胜出
在相应的dependency里面加上force="true"，就会强制使用这个低版本的jar，忽略其它间接依赖的高版本的；
参考 http://ant.apache.org/ivy/history/latest-milestone/ivyfile/dependency.html
Finally, the dependency element also supports an a force attribute (since 0.8), which gives an indication
to conflicts manager to force the revision of a dependency to the one given here.
See conflicts manager for details.
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