Add `fast_finish` to travis matrix
Add `fast_finish` to travis matrix

This means we don't have to wait for `allowed_failures` builds to
complete. It should save us ~10 minutes until we remove the windows
build from `allowed_failures`.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust