initialSlide should be int

Author: Wong Ho Chi Mark

src/slick.js

@@ -92,7 +92,7 @@ angular
               easing: scope.easing || 'linear',
               edgeFriction: scope.edgeFriction || 0.15,
               infinite: scope.infinite !== 'false',
-              initialSlide: scope.initialSlide || 0,
+              initialSlide: parseInt(scope.initialSlide) || 0,
               lazyLoad: scope.lazyLoad || 'ondemand',
               mobileFirst: scope.mobileFirst === 'true',
               pauseOnHover: scope.pauseOnHover !== 'false',