Always login to docker on travis instead of qualifying it

[lbergelson]

• It turned out we weren't logging in to dockerhub on the wdl test cases in travis because I misunderstood how the DOCKER_TEST flag was used.
• Now we unconditionally authenticate to dockerhub in all test shards instead of trying to pick only the relevant ones.

[lbergelson]

@kcibul @cmnbroad This should hopefully fix the problems you've both been seeing.

It turned out we weren't logging into docker on the wdl test branches because they don't set DOCKER_TEST but they still build docker images. I was confused about how that variable was used.

[kcibul]

LGTM -- let's see if the tests pass 🤣

[lbergelson]

Failures are non-deterministic so passing is a pretty weak signal.