The only masonry sealer I would ever use are absorbing water repellents., and, only when a distinct dry look was important in rainy weather, like cement brick veneer. Otherwise, what you can seal over from the outside, you can't on the beneath, which means a wet beneath in stone can grow stains you can't get to. Plus, chance of adding some slickness, possible yellowing, and having to reapply with needed stripper to prep. For me, it just ain't worth all that. BTW, we just laid 2,400 sf of that same travertine. Looks great like you have it.