Fixed a problem with the asset picker's footer color.

Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/112).

PS: I'd like to know if there's a better solution to this, so feel free to discuss [my modification to `CTAssetsPickerController`](https://github.com/chiunam/CTAssetsPickerController/pull/57), which is outside of this PR but a dependency and an important part of it.

/cc @aerych, @astralbodies