Enable IPv6 loopback for tests
Enable IPv6 loopback for tests

Summary:
Unit tests using the IPv6 loopback would fail on Travis with `[::1]:0: Cannot assign requested address`.

The issue is discussed at length here: https://github.com/travis-ci/travis-ci/issues/8711

Reviewed By: jstrizich

Differential Revision: D6914321

fbshipit-source-id: c247c6760dddcd875a67f7b31419bde974514f19

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