Fix for RSpec 3 deprecation warning

Using `update_index` matcher with RSpec 3 causes deprecation warning as follows:

```
Using `stub` from rspec-mocks' old `:should` syntax without explicitly enabling the syntax is deprecated. Use the new `:expect` syntax or explicitly enable `:should` instead. Called from /path/to/chewy/lib/chewy/rspec/update_index.rb:102:in `block (2 levels) in <top (required)>'.
```

Stubbing methods with `allow(obj).to receive(:message) { :value }` is backward-compatible (https://relishapp.com/rspec/rspec-mocks/v/2-14/docs/method-stubs), so it would be okay with RSpec 2 also.
Have no idea what was it for lol