Fix travis

[Diadlo]

This change is 

[Diadlo]

IPv6 is broken in Travis. Yey, it's not a toxcore issue!
travis-ci/travis-ci#8711

[sudden6]

not very familiar with the autotools part, but the rest

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Reviewed 4 of 5 files at r1, 8 of 8 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[robinlinden]

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Reviewed 4 of 5 files at r1, 8 of 8 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[e0ff]

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Reviewed 4 of 5 files at r1, 8 of 8 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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Comments from Reviewable

[nurupo]

I think the build flag and the define should have been named more explicitly as to hint that they enable/disable IPv6 only for tests, not for the entire toxcore.