Fixes the footer color for the assets picker.

Fixes [this issue](https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/112).

There was [a previous (failed) PR for this](https://github.com/wordpress-mobile/WordPress-iOS/pull/2717).  The author of CTAssetsPickerController [modified the library and proposed a new and supported way to change the picker's footer text color](https://github.com/chiunam/CTAssetsPickerController/pull/57).

/cc @aerych since you reviewed the original PR.