Fix bug with limitChildrenCount

When setting `limitChildrenCount` in config to a number larger than the amount of entries to format (in an array or object), the returned value from `formatio.ascii` contains a number of "undefined" strings up to the `limitChildrenCount`.

Now we only try to format as many entries as exist, up the `limitChildrenCount`.

I propose that we bump `formatio` to `1.1.1` after this pull request is merged, which will make it possible to fix https://github.com/cjohansen/Sinon.JS/issues/492, as that is dependent on `limitChildrenCount` configuration option working correctly.