chore(.appveyor.yml): remove "branch: master"
chore(.appveyor.yml): remove "branch: master"

avigad/lean,rlewis1988/lean,fpvandoorn/lean,Kha/lean,leanprover/lean,rlewis1988/lean,johoelzl/lean,johoelzl/lean,digama0/lean,dselsam/lean,johoelzl/lean,leodemoura/lean,Kha/lean,leanprover-community/lean,rlewis1988/lean,fpvandoorn/lean,leodemoura/lean,Kha/lean,digama0/lean,dselsam/lean,leanprover-community/lean,fgdorais/lean,dselsam/lean,leanprover-community/lean,leanprover/lean,johoelzl/lean,Kha/lean,Kha/lean,rlewis1988/lean,fpvandoorn/lean,digama0/lean,rlewis1988/lean,dselsam/lean,fgdorais/lean,Kha/lean,digama0/lean,avigad/lean,fgdorais/lean,johoelzl/lean,leanprover-community/lean,fgdorais/lean,fpvandoorn/lean,leanprover/lean,digama0/lean,leanprover/lean,dselsam/lean,rlewis1988/lean,leodemoura/lean,leanprover/lean,fpvandoorn/lean,avigad/lean,fpvandoorn/lean,leodemoura/lean,avigad/lean,leanprover-community/lean,avigad/lean,johoelzl/lean,avigad/lean,dselsam/lean,leanprover-community/lean,leanprover/lean,leodemoura/lean,fgdorais/lean,digama0/lean,fgdorais/lean,leodemoura/lean