fix: handle case where modprobe does not exist
fix: handle case where modprobe does not exist

This is possible when installing dokku within a docker image.

progrium/dokku,claytonbrown/dokku,claytonbrown/dokku,progrium/dokku,dokku/dokku,jimjag/dokku,dokku/dokku,jimjag/dokku,claytonbrown/dokku,progrium/dokku,jimjag/dokku,claytonbrown/dokku,progrium/dokku,claytonbrown/dokku,claytonbrown/dokku,progrium/dokku,jimjag/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,jimjag/dokku,dokku/dokku,jimjag/dokku,dokku/dokku,progrium/dokku,claytonbrown/dokku,jimjag/dokku,dokku/dokku,jimjag/dokku,progrium/dokku,dokku/dokku