Add missing rubocop dependencies

Rails and performance cops have been extracted to separate gems.

Note in the past we had to add these lines in order to activate Rails
cops:

```
Rails:
  Enabled: true
```

But we didn't do it, and so Rails cops were ignored.

With the new version, it's enough to require `rubocop-rails`.

.rubocop_basic.yml

@@ -1,4 +1,7 @@
-require: rubocop-rspec
+require:
+  - rubocop-performance
+  - rubocop-rails
+  - rubocop-rspec
 
 AllCops:
   DisplayCopNames: true