Reenable OSX travis builds #4576
Description
In the recentmost rustup (#4574) I marked OSX as an allowed failure on travis, due to rust-lang/cargo#7429. Cargo is being updated in rust-lang/rust#64774 , but I don't think we'll get access to that till the next nightly (it seems like rustup-toolchain-install-master uses fallback for tools.
We should reenable this in the next nightly.