Fixed issue with non-standard master branch name

Corrected issue by explicitly setting the branch name for HEAD.

Without this fix, Homebrew was unable to install `bfgminer` if a previous copy of `bfgminer` HEAD had been installed and then uninstalled.
Merge branch 'refs/heads/bfgminer'

* refs/heads/bfgminer:
  Fixed issue with non-standard master branch name