Fix TextBlock not accepting hex colors

As mentioned in #532, the sample now starts properly without crashing. However, the sample prints an object string and I'm not sure if this is related to my fix (Probably not).

![selection_022](https://f.cloud.github.com/assets/2042399/1958769/ac5ad5ec-821f-11e3-8cb2-bc815bce8c9f.png)

If I change [samples/good-clock.rb#L15](https://github.com/shoes/shoes4/blob/master/samples/good-clock.rb#L15) to use `span` instead of `strong`, it doesn't show an object string.