travis: bump rustc to 1.22.1
travis: bump rustc to 1.22.1

Signed-off-by: Igor Gnatenko <c869b0bfe2cf1da26c2efd33f6d6d74ddcbbdd2b@gmail.com>

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