Require `rspec/collection_matchers` by default

Currently `rspec-collection_matchers` gem is specified as a runtime dependency but it's not loaded, so `have(...).error_on(...)` fails as the following:

```
     Failure/Error: it { should have(1).error_on(:name) }
     NoMethodError:
       undefined method `have' for #<RSpec::ExampleGroups::User_2::Name::WhenNameIsNotPresent:0x007fc3d72a3648>
```

I think `2-99-maintenance` branch also needs to require it to suppress the deprecation warning.