Make Travis faster
Make Travis faster

* Remove sudo: required
* Remove rust=beta build on OSX
* Remove rust=nightly builds

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