Add guidance for %i/%I to create arrays of symbols

Ruby 2 gives us `%i` and `%I` for arrays of symbols (with/out interpolation, respectively). Add some guidance around using them.

- Use %i() for arrays of symbols (ruby 2!)

```ruby
  # bad
  [:one, :two, :three]

  # good
  %i(one two three)
```

- Use %I() for arrays of symbols which require interpolation

```ruby
  # bad
  a = "two"
  [:one, a.to_sym, :three] # => [:one, :two, :three]

  # good
  a = "two"
  %I(one #{a} three) # => [:one, :two, :three]
```