Make drush_drupal_version() pull from Boot classes

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With #342 and other enchancements, we have Boot classes that separate out the version-specific logic when bootstrapping D6/D7/D8 (and Backdrop).

One area where we're having trouble in Backdrop land is that drush_drupal_version() is hard-coded to look in particular areas for the VERSION constant. As Backdrop wants to identify itself as D7 (for now), but the logic is hard-coded, we are unable to make drush recocognize Backdrop without some pretty sad hacking (like making an empty file at includes/bootstrap.inc that contains a VERSION constant).

Separating out the logic for different versions of Drupal is what Boot classes were intended for, so could we move the logic of drush_drupal_version() into these classes?

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Hopefully this isn't too presumptious, but here are PRs for 7.x and master:

7.x: #1725
master: #1726

If they need work I can just turn into a single PR if that's preferred.

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Thanks @weitzman and @greg-1-anderson!