Invalid symbol in _backend_gdk.c if numpy version < 1.7

PR #3665 added the following to `_backend_gdk.c` (lines 40-43):

```
array = (PyArrayObject*)
        PyArray_New(&PyArray_Type, 3, dims, NPY_UBYTE, strides,
                    (void*)gdk_pixbuf_get_pixels(gdk_pixbuf), 1,
                    NPY_ARRAY_WRITEABLE, NULL);
```

The flag `NPY_ARRAY_WRITEABLE` is not valid for numpy versions < 1.7, the equivalent flag is `NPY_WRITEABLE`.
