Activate Travis in PR check

[andreyvelich]

We should activate Travis check in presubmit and postsubmit.

Also goveralls should report status for coveralls (See comment: https://github.com/kubeflow/pytorch-operator/pull/293#issuecomment-666045781) and we should migrate to travis-ci.com (See comment: https://github.com/kubeflow/pytorch-operator/pull/293#issuecomment-674088221).

/cc @johnugeorge @gaocegege /priority p1

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