doc(automatic_builds): add how to prevent a build on Travis-ci
doc(automatic_builds): add how to prevent a build on Travis-ci

[skip ci]
codyroux/lean0.1,Kha/lean,fgdorais/lean,soonhokong/travis_test,UlrikBuchholtz/lean,soonhokong/lean,Kha/lean,johoelzl/lean,rlewis1988/lean,johoelzl/lean,leanprover/lean,dselsam/lean,leanprover-community/lean,leanprover-community/lean,javra/lean,leanprover-community/lean,eigengrau/lean,sp3ctum/lean,eigengrau/lean,dselsam/lean,soonhokong/lean,leanprover/lean,soonhokong/travis_test,htzh/lean,digama0/lean,fpvandoorn/lean2,Kha/lean,avigad/lean,codyroux/lean0.1,soonhokong/travis_test,fgdorais/lean,avigad/lean,soonhokong/lean-osx,digama0/lean,rlewis1988/lean,johoelzl/lean,Kha/lean,leodemoura/lean,rlewis1988/lean,fpvandoorn/lean2,javra/lean,Kha/lean,avigad/lean,fgdorais/lean,levnach/lean,sp3ctum/lean,leanprover/lean,levnach/lean,soonhokong/lean-osx,soonhokong/lean,soonhokong/lean,sp3ctum/lean,sp3ctum/lean,soonhokong/lean-osx,leanprover-community/lean,javra/lean,sp3ctum/lean,avigad/lean,digama0/lean,avigad/lean,avigad/lean,leodemoura/lean,fpvandoorn/lean,digama0/lean,leanprover/lean,javra/lean,leodemoura/lean,eigengrau/lean,fpvandoorn/lean,fpvandoorn/lean2,levnach/lean,fpvandoorn/lean2,soonhokong/lean-windows,leanprover/lean,htzh/lean,soonhokong/lean,soonhokong/lean-windows,leodemoura/lean,dselsam/lean,leodemoura/lean,codyroux/lean0.1,eigengrau/lean,soonhokong/lean-windows,fpvandoorn/lean,htzh/lean,c-cube/lean,soonhokong/lean-windows,dselsam/lean,fpvandoorn/lean,UlrikBuchholtz/lean,rlewis1988/lean,dselsam/lean,leodemoura/lean,dselsam/lean,soonhokong/lean-osx,rlewis1988/lean,UlrikBuchholtz/lean,fpvandoorn/lean,codyroux/lean0.1,c-cube/lean,leanprover-community/lean,fpvandoorn/lean2,fgdorais/lean,soonhokong/lean-osx,levnach/lean,UlrikBuchholtz/lean,fpvandoorn/lean,UlrikBuchholtz/lean,c-cube/lean,johoelzl/lean,c-cube/lean,johoelzl/lean,digama0/lean,soonhokong/travis_test,eigengrau/lean,leanprover-community/lean,javra/lean,rlewis1988/lean,c-cube/lean,levnach/lean,fgdorais/lean,Kha/lean,fgdorais/lean,johoelzl/lean,htzh/lean,htzh/lean,soonhokong/lean-windows,leanprover/lean,digama0/lean