Slow Cooker Shredded Chicken in Yogurt Sauce

½ cup fat-free Greek yogurt

1 (6 ounce) can tomato paste

1 onion

chopped

4 cloves garlic

2 tablespoons curry powder

1 tablespoon grated fresh ginger root

1 ½ pounds skinless chicken breasts

salt and freshly ground black pepper to taste

1 teaspoon sriracha sauce

or to taste