Use Rubocop's Include
instead of manually checking the matcher in controllers specs.

BeSuccessMatcher now supports following examples:

```
expect(response).to be_success
expect(response).to_not be_success
expect(response).not_to be_success

is_expected.to be_success
is_expected.to_not be_success
is_expected.not_to be_success
```

Prevent using `be_success` call in controller specs to avoid
getting following deprecation warning:

```
DEPRECATION WARNING: The success? predicate is deprecated and
will be removed in Rails 6.0.
Please use successful? as provided by Rack::Response::Helpers.
```

Stop checking the file location programmatically.

The RSpec/FilePath cop checks that a spec file's path is correct, but
only if the file describes a constant. We want to check, for any file
with a top-level `describe`, whether the file path ends in
`_spec.rb`. We don't care what comes before that; just that it will be
executed by RSpec at all.