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

bin/ruby-build

@@ -829,7 +829,7 @@ require_llvm() {
             colorize 1 "TO FIX THE PROBLEM"
             echo ": Install Homebrew's llvm package with this"
             echo -n "command: "
-            colorize 4 "brew install llvm"
+            colorize 4 "brew tap homebrew/versions ; brew install llvm35"
             echo
           } >&3
           return 1