Enable Travis on branches like "r0.19"

Merge nix-rust#1307

1307: Release v0.19.0. r=kamalmarhubi a=qwandor-google

I'd like to get a new version of the crate released including nix-rust#1301 so that I can get the vsock crate building on Android, and get both imported into the Android tree.

Co-authored-by: Andrew Walbran <qwandor@google.com>

Merge nix-rust#1277

1277: Release version 0.18.0 r=asomers a=asomers



Co-authored-by: Alan Somers <asomers@gmail.com>

Release 0.17.0

Merge nix-rust#1162

1162: Release v0.16.0 r=asomers a=asomers



Co-authored-by: Alan Somers <asomers@gmail.com>

Release v0.13.1

Release v0.12.1

Merge nix-rust#1081

1081: Release v0.14.1 r=asomers a=asomers



Co-authored-by: Alan Somers <asomers@gmail.com>

Release v0.11.1

Merge nix-rust#1061

1061: Release v0.14.0 r=asomers a=asomers



Co-authored-by: Alan Somers <asomers@gmail.com>