Improved page.image method

Now **page.image** gives always the first element in **page.images**

```ruby
page = MetaInspector.new 'https://twitter.com/astro_reid/status/527799661823008768'
page.image # https://pbs.twimg.com/profile_images/1431054149/image_bigger.jpg
```

With this change, its easier to get images from URLs.

Hope you like it :smile: 
I think this comment is not needed, we are already discussing about it on https://github.com/jaimeiniesta/metainspector/issues/90