Build definiiton and junit publisher

I have a build.xml in my project, which can be run sucessfully.

Now I add a line
"<import file="BuildToolkitTaskDefs.xml"/>
to my build.xml.
I hope that the result of junit can be published to build result.

But build log always:

D:\Jazz_M3a_workspace\dev_client\jihan\Samples\BuildToolkitTaskDefs.xml:14: java.lang.UnsupportedClassVersionError: com/ibm/team/build/ant/task/ArtifactFilePublisherTask (Unsupported major.minor version 49.0)

Is there any steps I have missed?