Boosted .reveal-modal z-index to display it on top of its background.

`.reveal-modal-bg` was showing on top of the modal due to a change in its `z-index` value which made it greater than the one specified for the `.reveal-modal` class (`100` > `41`).
As I suppose the change from `40` to `100` has a reason I'm not aware of, I decided to boost the modal's `z-index` to `101`.