Move Travis Windows build to allowed failures
Move Travis Windows build to allowed failures

Until the remaining issues are fixed. This also enabled `fast_finish`.
It will finish even if the windows build is still running.

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