Update instructions to install LLVM 3.5 using Homebrew

Homebrew bumped the version of LLVM installed with `brew install llvm`
from 3.5.1 to 3.6.0. So now users will have to tap homebrew-versions and
run `brew install llvm35` in order to get the version necessary to
install recent versions of Rubinius. The 3.5-specific `llvm-config`
becomes `llvm-config-3.5` accordingly.

https://github.com/Homebrew/homebrew/commit/0196a069a01e455b61778d804ebb81f46e7c31ff