chore(travis): use gcov-4.8 on ubuntu
chore(travis): use gcov-4.8 on ubuntu

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