GitLab

* Packaging improvements for nightlies

- Instructions for how to install stable and master binaries,
  without hardcoding wheel filenames
- Turn on Python 2 build (numba is only needed for testing)
- Make builds a little more hermetic
- Prefer building against nightly CPU-only torch build
- Don't install all of requirements.txt, it's not necessary
- Updates for OS X too, including portability issues
Signed-off-by: Edward Z. Yang <ezyang@fb.com>