fixes CodeIgniter.gitignore deleting a critical system file

After spending a lot of time finding why my app won't run, just noticed that the gitignore I copied introduced a deletion/ignore rule of an internal folder used by the framework.
The current gitignore file deletes the folder `/system/Cache/*` and causes the following error when running the App from the CLI or the web server : `Class "CodeIgniter\Cache\CacheFactory" not found `; making the app unable to run.

I added the following rule to fix this : `!system/Cache/*`.

Note : this won't fix the bug if the system folder is renamed