Fix macos build (#15873)

Fix macos build (#15873)

Summary:
macos builds are broken now with the following error:
```
/usr/local/Homebrew/Library/Homebrew/config.rb:39:in `initialize': no implicit conversion of nil into String (TypeError)
	from /usr/local/Homebrew/Library/Homebrew/config.rb:39:in `new'
	from /usr/local/Homebrew/Library/Homebrew/config.rb:39:in `<top (required)>'
	from /usr/local/Homebrew/Library/Homebrew/vendor/portable-ruby/2.3.7/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /usr/local/Homebrew/Library/Homebrew/vendor/portable-ruby/2.3.7/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /usr/local/Homebrew/Library/Homebrew/global.rb:25:in `<top (required)>'
	from /usr/local/Homebrew/Library/Homebrew/brew.rb:13:in `require_relative'
	from /usr/local/Homebrew/Library/Homebrew/brew.rb:13:in `<main>'
Exited with code 1
```

No recent commits look suspicious, and I can even reproduce locally on my macbook, so it might be related to some new `brew` updates. Empirically, calling `brew update` first seems to fix this.

Example error build: https://circleci.com/gh/pytorch/pytorch/534392?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15873

Differential Revision: D13608019

Pulled By: soumith

fbshipit-source-id: 1499cb5246929e275a11ca6fccef6ef32918e45e