build backend in travis ci

[chenzhiwei]

In the docker image, there are three binaries, but travis ci only builds two of them.

Adding the backend binary to travis ci to improve the ci build.

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This change is 

[k8s-ci-robot]

Hi @chenzhiwei. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[gaocegege]

Thanks for your contribution!

Yeah, I think we should build the dashboard in Travis CI, too.

[gaocegege]

/lgtm
/approve

[gaocegege]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gaocegege

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[coveralls]

Coverage remained the same at 64.745% when pulling 228ad4c on chenzhiwei:travis-backend into befef38 on kubeflow:master.