Use "which" command exit status instead of the output: issue #106

Fix for issue #106 

Checking the presence or absence of the which's output in .zshrc.

    [[ -s "${GVM_DIR}/bin/gvm-init.sh" && ! $(which gvm-init.sh) ]] && source "${GVM_DIR}/bin/gvm-init.sh"


The "which" of zsh always prints a message to stdout even if a command is not found.

    % which gvm-init.sh
    gvm-init.sh not found

Therefore, should be check the exit status of the command.