Make migration dependant on `cms`.

Fixes migration errors when running tests when `djangocms_text_ckeditor` is listed *before* `cms` in `INSTALLED_APPS` (as advised in the docs).

```
Creating test database for alias 'default'...
FATAL ERROR - The following SQL query failed: ALTER TABLE "cmsplugin_text" ADD CONSTRAINT "cmsplugin_ptr_id_refs_id_34ad5108" FOREIGN KEY ("cmsplugin_ptr_id") REFERENCES "cms_cmsplugin" ("id") DEFERRABLE INITIALLY DEFERRED;
The error was: relation "cms_cmsplugin" does not exist

Error in migration: djangocms_text_ckeditor:0001_initial
DatabaseError: relation "cms_cmsplugin" does not exist
```