The BBC's Simon Calder addressed this very problem on the 'BBC Breakfast' programme this morning. Basically, he said 'No, you're not entitled to a refund'. Reasoning behind this, say the travel companies, is that bombings have gone on in Turkey for several years now, and that you, (the public), should be aware of that before you book your holiday. If the FO were to issue a travel warning against going to Turkey, that would change things, but otherwise... And the FO aren't likely to do that, since they'd prefer to maintain a cosy relationship with Turkey. (Think of the bombings in Spain...same thing).
Going by past experiences, what's happened in Marmaris is pretty unlikely to happen again during your trip.