Cron issue with Drush 8
I would suggest that instead of calling cd within your script, you either set up drush aliases or pass the --root and --uri parameters to the cron request. 

/home/myuser/.composer/vendor/bin/drush -v --root=/home/myuser/public_html --uri=fr.mysite.com core-cron


That should make sure that drush is running in the right context. 

You can also make sure that drush is running with the php that you would like by setting the DRUSH_PHP environment variable.