Auto merge of #780 - rillian:llvm4, r=emilio
Auto merge of #780 - rillian:llvm4, r=emilio

Drop llvm@3.9 versioning for homebrew.

Homebrew now defaults to llvm 4.0.x so there's no need to specify a specific version to get compatibility, unless we specifically want people using 3.9.

emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen