chore: don't run CI on the ivy-2019 branch

[jelbourn]

No description provided.

[crisbeto]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: e2e_tests" is failing
     status "ci/circleci: tests_browserstack" is pending
     status "ci/circleci: tests_local_browsers" is pending
     status "ci/circleci: tests_saucelabs" is pending

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

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