Add sparse option to slug index.

Without this option, db:mongoid:create_indexes fails because of multiple null values when user don't already have slugs.

```
rake aborted!
The operation: #<Moped::Protocol::Command
  @length=83
  @request_id=7
  @response_to=0
  @op_code=2004
  @flags=[]
  @full_collection_name="project_development.$cmd"
  @skip=0
  @limit=-1
  @selector={:getlasterror=>1, :w=>1}
  @fields=nil>
failed with error 11000: "E11000 duplicate key error index: project_development.users.$_slugs_1  dup key: { : null }"

See https://github.com/mongodb/mongo/blob/master/docs/errors.md
```

http://docs.mongodb.org/manual/core/index-unique/