No need to show Twitter image when OpenGraph is showing

Both the description within the PR, and that next to the `image()` method suggests that it doesn't need to be called when OpenGraph is also showing. Using the Twitter Card Validator confirms that when `twitter:image:src` isn't present, it falls back to using `og:image`.
Merge pull request #151 from GaryJones/patch-1

No need to show Twitter image when OpenGraph is showing