Refine Drupal detections

[markhalliwell]

Thanks for the release!

Unfortunately, I didn't account that the png images wouldn't show up in the toolbar. Guess they have to be ico format.

Also, I noticed a couple discrepancies:

• Incorrect version reported when there is no meta tag (needs new D7 JS test)
• Both Drupal8 and Drupal showing on D8 sites.

Here are some refinements.