feat(coverage): add coveralls support

[jimexist]

fixes #155

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This change is 

[k8s-ci-robot]

Hi @jimexist. Thanks for your PR.

I'm waiting for a kubernetes 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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[jimexist]

@jlewi I think you'll need to enable this repo in coveralls.io (just flip the toggle). After that I can add the badge as well.

[jlewi]

@jimexist I don't see the repo in coveralls.io; I suspect it might be because I'm not a member of the tensorflow organization.

@jhseu Is coveralls.io something you can enable for this repo?

[jimexist]

@jlewi yes we need someone from tensorflow team to add repo in coverall.io

[jlewi]

Haven't forgotten about this. I'm guessing Jonathan is out for the holiday. Next week if he hasn't chimed in I'll ping him again and see about getting it enabled.

Thanks for your patience.

[jlewi]

I pinged @jhseu to see about getting this enabled.

[jimexist]

@jlewi is this enabled yet?

[jlewi]

@jhseu enabled coveralls.io for the tensorflow.org.

Is there anything else that needs to be done? When I look in coveralls.io I don't see the tensorflow/k8s repository.

@jhseu said that we can only enable coveralls.io in public repo mode;we can't enable it for reading private repos. Do you know what this means?

[jimexist]

@jlewi i think org owner needs to add org into coveralls.io and also enable the repo for coveralls (i.e. toggle the button)

like this:

[jimexist]

migrated to #232 - sorry that I accidentally closed the branch @jlewi