Autocomplete Basic fix for issue #1381

This is only a basic fix that removes `this.`, `@` and `self.` from the snippets if already in place. More can be added by setting in the zatanna initialization in [SpellView.coffee#L101](https://github.com/codecombat/codecombat/blob/master/app/views/play/level/tome/SpellView.coffee#L101) the value of `languagePrefixes` similar to this:

```coffee
@zatanna = new Zatanna @ace,
      languagePrefixes: 'this.,@,self.'
      liveCompletion: aceConfig.liveCompletion ? true
      completers:
        keywords: false
```