Emscripten download fails silently on travis

[svenstaro]

I'm reporting this for rand. Basically the issue is that cargo web prepare-emscripten fails on master but not in PRs. Super weird but doesn't appear to be caching related.

Failed job example: https://travis-ci.org/rust-random/rand/jobs/476157401

Discussions: https://github.com/rust-random/rand/pull/684 and https://github.com/rust-random/rand/pull/671

Maybe you have any idea what's going on.