Reorder builds to improve build times
Reorder builds to improve build times

The main issue is that all iOS builds are in a block and there are only 5
simultaneous builds allowed by our Travis CI plan. So if there's a backlog
of iOS builders, then the entire build process stalls. So instead of having
them all clumped together, distribute the Mac and iOS builds throughout the
build pipeline.

nix-rust/nix,asomers/nix,Susurrus/nix,nix-rust/nix,carllerche/nix-rust,carllerche/nix-rust,MarkusJais/nix-rust,geofft/nix-rust,xd009642/nix,xd009642/nix,Susurrus/nix,xd009642/nix,Susurrus/nix,MarkusJais/nix-rust,asomers/nix,geofft/nix-rust,MarkusJais/nix-rust,carllerche/nix-rust,geofft/nix-rust