Retrieve versions of tools installed via brew.

def _get_brew_versions():
    """Retrieve versions of tools installed via brew.
    """
    from bcbio import install
    tooldir = install.get_defaults().get("tooldir")
    brew_cmd = os.path.join(tooldir, "bin", "brew") if tooldir else "brew"
    try:
        vout = subprocess.check_output([brew_cmd, "list", "--versions"])
    except OSError:  # brew not installed/used
        vout = ""
    out = {}
    for vstr in vout.split("\n"):
        if vstr.strip():
            parts = vstr.rstrip().split()
            name = parts[0]
            v = parts[-1]
            out[name] = v
    return out