vagrant-dev : in bootstrap.sh, the COMPOSER_UPDATE variable don't works.

similar to #736 (closed)

in Vagrant/.../bootstrap.sh, the following code don't works.

declare COMPOSER_UPDATE=0

if [[ "${COMPOSER_UPDATE}" ]]; then
    echo "---------------> composer UPDATE"
    # ---> the previous line is always displayed
    (...)
fi

see: https://unix.stackexchange.com/questions/105935/understanding-boolean-operators-in-bash-script