Changed PARAMETERS constant to be an array of symbols, instead of an array of strings converted into symbols.

```ruby
# In irb:
2.1.5 :001 > %i{ hello world }
 => [:hello, :world]
```

The `%i` prefix tells ruby that anything after `{` or `[` will be an array of symbols. 

I've simply removed changed the CONSTANTS parameter list from being an array of strings mapped into symbols to simply an array of symbols.