Fix rubocop violations in generated comment.rb

* Remove extraneous space after class declaration
* Use Ruby 1.9+ hash syntax (since Rails 4+ requires Ruby 1.9+ anyway)
* [default_scope expects a block, not a proc](https://github.com/bbatsov/rubocop/blob/master/lib/rubocop/cop/rails/default_scope.rb)
* Add space after #

Previous rubocop output:
```
Offenses:

app/models/comment.rb:2:1: C: Extra empty line detected at class body beginning.
app/models/comment.rb:5:28: C: Use the new Ruby 1.9 hash syntax.
  belongs_to :commentable, :polymorphic => true
                           ^^^^^^^^^^^^^^^
app/models/comment.rb:7:17: C: default_scope expects a block as its sole argument.
  default_scope -> { order('created_at ASC') }
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
app/models/comment.rb:11:3: C: Missing space after #.
  #acts_as_voteable
  ^^^^^^^^^^^^^^^^^
```