The moisture can come from two possible areas. The travertine tiles are somewhat porous and must be sealed about twice per year. Check into marble and stone sealers at your tile dealer. The other possibility is that the shower pan was not properly sealed, or the seal has failed. This may require rebuilding the shower pan. I would start by sealing the tile and grout first to try to elimnate that possibility. If the leak persists, you will probably need professional assistance.