fix(.travis.osx.yml): explicitly turn on multi-thread support
fix(.travis.osx.yml): explicitly turn on multi-thread support

avigad/lean,leanprover/lean,UlrikBuchholtz/lean,soonhokong/lean-windows,dselsam/lean,htzh/lean,fgdorais/lean,fgdorais/lean,eigengrau/lean,levnach/lean,dselsam/lean,digama0/lean,javra/lean,leanprover-community/lean,levnach/lean,levnach/lean,eigengrau/lean,leodemoura/lean,leodemoura/lean,fpvandoorn/lean,rlewis1988/lean,soonhokong/lean-osx,fpvandoorn/lean2,rlewis1988/lean,leanprover-community/lean,soonhokong/lean,leanprover/lean,javra/lean,rlewis1988/lean,johoelzl/lean,c-cube/lean,c-cube/lean,digama0/lean,Kha/lean,leanprover-community/lean,avigad/lean,soonhokong/lean-osx,leanprover-community/lean,eigengrau/lean,UlrikBuchholtz/lean,Kha/lean,Kha/lean,avigad/lean,johoelzl/lean,digama0/lean,fgdorais/lean,soonhokong/lean,avigad/lean,johoelzl/lean,johoelzl/lean,c-cube/lean,digama0/lean,levnach/lean,fpvandoorn/lean,fpvandoorn/lean,avigad/lean,sp3ctum/lean,fpvandoorn/lean,htzh/lean,fpvandoorn/lean2,rlewis1988/lean,rlewis1988/lean,sp3ctum/lean,javra/lean,htzh/lean,c-cube/lean,Kha/lean,sp3ctum/lean,leodemoura/lean,sp3ctum/lean,fpvandoorn/lean2,dselsam/lean,htzh/lean,soonhokong/lean-windows,fpvandoorn/lean2,UlrikBuchholtz/lean,UlrikBuchholtz/lean,UlrikBuchholtz/lean,soonhokong/lean-windows,digama0/lean,johoelzl/lean,fgdorais/lean,soonhokong/lean-windows,javra/lean,soonhokong/lean-osx,c-cube/lean,fpvandoorn/lean,leodemoura/lean,levnach/lean,johoelzl/lean,leanprover-community/lean,eigengrau/lean,leanprover/lean,digama0/lean,soonhokong/lean-osx,avigad/lean,soonhokong/lean-windows,soonhokong/lean,rlewis1988/lean,htzh/lean,Kha/lean,leanprover-community/lean,leodemoura/lean,leodemoura/lean,dselsam/lean,fpvandoorn/lean2,eigengrau/lean,soonhokong/lean,fpvandoorn/lean,leanprover/lean,leanprover/lean,dselsam/lean,fgdorais/lean,soonhokong/lean,javra/lean,leanprover/lean,sp3ctum/lean,fgdorais/lean,soonhokong/lean-osx,Kha/lean,dselsam/lean