stops zsh compinit from complaining about insecure directories

removes the group write permissions during the make install phase

This commit is necessary, in addition to the changes brought by #1226, in order to avoid the following error while running zsh:

    $ zsh
    zsh compinit: insecure directories, run compaudit for list.
    Ignore insecure directories and continue [y] or abort compinit [n]?

    $ compaudit
    There are insecure directories:
    /usr/local/zsh/share/zsh
    /usr/local/zsh/share/zsh/5.0.5