Update the build image in README.md
Update the build image in README.md

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