set background color to labels for iOS7

Hi, thank you for this nice tutorial about RubyMotion.
I am studying RubyMotion with it now, but in chapter 7 I did not get the same UI result as showed in the article. To be precise, each UILabel were showed but its background color was black, so I did not see whether I got the right result.

This commit simply fixes the background color of each UILabel to white.