Fixed stale brew tap, added LLVMDIR workaround

[lisaong]

• homebrew/science/openblas is not longer valid. This has migrated over to homebrew-core which is up to the latest release of OpenBlas
  • The version in OpenBLASSetup.cmake is hardcoded, which I'm not super happy with, but I can't think of a cleaner way that doesn't require the user to setup a symlink.
• for a clean setup on macOS sierra, CMake can't find LLVM. Tried creating a symlink in /user/local/Cellar/llvm to llvm@3.9 to no avail.
• cleaned up troubleshooting instruction that no longer applies

[msftclas]

All CLA requirements met.

[kernhanda]

LGTM.