Use Composer to manage sub-theme Bootstrap LESS dependencies when using LESS starterkit
composer require twbs/bootstrap ^3.3.6 --prefer-dist -vvv

Drupal Bootstrap requires 3.x.x the caret will stop us tripping into v4.

We don't want to include any git files so we do --prefer-dist.

This downloads the latest bootstrap source to vendor/twbs.

I can expose this dir to my subtheme by editing:

/themes/custom/MYSUBTHEME/less/bootstrap.less

Now when I run composer update I will get the latest bootstrap less in my subtheme.