Activate Site Feature through REST API

I'm trying to activate site level feature by using REST API in SharePoint Hosted App. I've given full control at Site Collection & Site level with App only Calls. But I'm getting "Access Denied Error" with 403 status. 

function bindButtonClick() {

            var featuresURI = hostUrl + "/_api/web/features/add('a7a2793e-67cd-4dc1-9fd0-43f61581207a')"; 
                $.ajax({
                    url: featuresURI,
                    method: 'POST',
                    headers: {
                        "Accept": "application/json;odata=verbose",
                        "X-RequestDigest": $("#__REQUESTDIGEST").val()
                    },
                    success: function (data) {
                        console.log("Feature Activated");
                    },
                    error: function (data) {
                        console.log("error: " + JSON.stringify(data));
                    }
                });
            }

Error:

Failed to load resource: the server responded with a status of 403 (Forbidden)
error: {"readyState":4,"responseText":"{\"error\":{\"code\":\"-2147024891, System.UnauthorizedAccessException\",\"message\":{\"lang\":\"en-US\",\"value\":\"Access denied. You do not have permission to perform this action or access this resource.\"}}}","status":403,"statusText":"Forbidden"}