release pre-built binaries that work on travis

[gnzlbg]

It would be awesome if we would have a script like rustup does for installing itself, that would just download pre-compiled binaries of cargo-travis that "just work" so that one does not have to compile it.

Github supports hosting binaries on releases (when a build is tagged as such), so maybe the script could just download the binary from here. The x86_64-unknown-linux-gnu target with everything statically linked would probably "just work".