Fixes (on centos6):
which: no flock in (/home/linuxbrew/.linuxbrew/bin:/home/linuxbrew/.linuxbrew/sbin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin)
Ruby first gained flock in 1.8.7, which is a problem since we're using
this lock utility in `vendor-install` in order to install a newer Ruby.
Fortunately, Python 2.3(!) has flock support.
Since #292, HOMEBREW_CACHE was moved to a per-user directory. This makes
it unsuitable to store global lock files on multiple users environment.
Therefore, introducing a global lock directory `/Library/Lock.d` to
store lock files from formula lockers as well as `brew update`.