Please use a cache to retrieve llvm tarballs

[sylvestre]

We, LLVM, have a few issues with people downloading llvm or clang every run of the CI.

See http://lists.llvm.org/pipermail/llvm-dev/2016-May/100303.html
or http://lists.llvm.org/pipermail/llvm-dev/2017-May/113269.html

I know you are not using apt.llvm.org but this still has an important impact to download 216mo at each run of the CI

Done here:
https://github.com/rust-lang-nursery/rust-bindgen/blob/master/ci/before_install.sh#L13

[emilio]

Sure, happy to, thanks for the heads-up.

I'll try to make it work using https://docs.travis-ci.com/user/caching/#Arbitrary-directories, but let me know if there's any repo I could look at to verify it works.