Remove option that caused exception
Remove option that caused exception

Fixes

```
Invalid option :index provided to relation :comments. Valid options are:
as, autosave, dependent, foreign_key, order, class_name, extend,
inverse_class_name, inverse_of, name, relation, validate.
(Mongoid::Errors::InvalidOptions)
```

when running `rake`

All specs now pass.

nofxx/mongoid-rspec,xronos-i-am/mongoid-rspec,bartes/mongoid-rspec,mongoid-rspec/mongoid-rspec