"The Ruins," based on Scott B. Smith’s novel and directed by Carter Smith, avoids common pitfalls of horror films by resisting the urge to go overboard with its thrills and "monster." Films that rely on excessive spectacle often end up being more comical than frightening; “The Ruins” steers clear of this trap, especially noticeable in how it handles the ending, which differs from the book and feels like an improvemen…