Travis: use LLVM3.9 on OS X

[JohanEngelen]

Let's see if this works (so that we can release with LLVM3.9 on OS X).

[JohanEngelen]

Homebrew does not have llvm3.9 yet.

See:
Homebrew/homebrew-core#4579
https://github.com/Homebrew/homebrew-versions/pull/1384

[JohanEngelen]

Instead of waiting for homebrew, I rewrote travis.yml to download the binaries from LLVM directly and cache them like we do for the Linux builds. This appears to speed up the build a lot (~39min -> ~30min), possibly because LLVM's binaries have all assertions disabled?

[JohanEngelen]

(green for mac with LLVM3.9 btw, nice :-))

[kinke]

possibly because LLVM's binaries have all assertions disabled

Seems like it, as we don't have assertions enabled on Linux as well. I must say I really prefer CI builds with enabled assertions; people keep complaining about LDC crashing, and most of these issues (and others!) would be caught by enabled assertions. I would even prefer alphas and betas to ship with assertions too.
Does CircleCI have assertions enabled or is it just AppVeyor now?

[JohanEngelen]

Pretty sure that CircleCI also does not have assertions enabled...

Hm.

[JohanEngelen]

Already merged in release-1.1.0 branch.