Only run AppVeyor on r+, try and the master branch
Only run AppVeyor on r+, try and the master branch

As it is right now, there is only one worker available in the `rust-lang-libs`
AppVeyor project and there are other repos as well that we share this worker
with. This has been a problem for us because we sometimes hit a bors timeout if there
are too many builds queued up.

To improve the situation, I think we could try to use AppVeyor a bit less
often. The average PR is not going to break windows related things anyway, so
it should be fine to run it on r+/try/master only.

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