Change update_homebrew script to work with single-file homebrew formulas.
Change update_homebrew script to work with single-file homebrew formulas.

BUG=
R=ricow@google.com

Review URL: https://codereview.chromium.org//654653007

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@41125 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/homebrew-dart,dart-lang/homebrew-dart,dart-lang/homebrew-dart