Fix a bug that somwtimes colorize option doesn't work

If colorize option is declared like this:

```ruby
Rack::SpyUp.config do |config|
  config.colorize = false
end
```

Then it'll just blow up and you'll get:

```
NoMethodError: undefined method `colorize=' for #<Rack::SpyUp::Configuration:0x00000000ff15e0>
```

This commit fixes this bug and now `colorize` options can be configured properly on Rails.