build: use rxjs@6.0.0-terrific-rc.3

[robwormald]

Bump to latest version of rxjs to eliminate type conflicts

[ngbot]

I see that you just added the PR action: merge label, but the following checks are still failing:
     missing required label: "cla: yes"
     status "code-review/pullapprove" is pending
     status "ci/circleci: lint" is pending
     status "ci/circleci: build" is pending
     status "continuous-integration/travis-ci/pr" is pending

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