add "Theme - Centurion Blue" as previous_name of "Theme - Centurion"

I removed [my fork](https://github.com/fnkr/ST-Centurion-Blue) from package control because the original theme now [can be configured](https://github.com/allanhortle/Centurion#customisation) to use blue instead of green. I think it makes sense to automatically switch the users to the original theme.

https://github.com/fnkr/ST-Repository/commit/9ea68d6e748a13402b852c0157698272273bf905

@allanhortle