feat(download/index.md): remove ubuntu 15.10, add ubuntu 16.04
feat(download/index.md): remove ubuntu 15.10, add ubuntu 16.04
leanprover/leanprover.github.io,avigad/leanprover.github.io,avigad/leanprover.github.io,avigad/leanprover.github.io,leanprover/leanprover.github.io,leanprover/leanprover.github.io