Run 'drush make-verify' on Travis-CI to make sure ALL .make files will work on Drupal.org

The Panopoly 1.3 release failed to build because manualcrop needed some libraries that weren't in the Drupal.org Whitelist, and I forgot to check. So that I can never make this stupid mistake again, let's add this to our Travis-CI tests!

The code will be something like:

drush dl -y drupalorg_drush-7.x-1.x-dev --destination=$HOME/.drush
drush verify-makefile drupal-org.make
find modules/ -name \*.make -exec drush verify-makefile \{\} \;

Note 1: The find bit is required to check all .make files since drush verify-makefile won't recurse into sub-make files, which wouldn't catch the manualcrop issues.

Note 2: I'm not sure the find will correctly return an error code that Travis-CI will detect as failure, so this will need some experimentation. And, unfortunately, I can't do it right now, because the build is broken by Drush installation per an upstream issue: https://github.com/drush-ops/drush/issues/601

Anyway, I'm looking forward to the build pretecting Panopoly from me. :-)