Update Administration Menu to admin_menu-7.x-3.0-rc6

On my local dev site, I did the following:

1. Disabled 'navbar'
2. Enabled 'admin_menu'
3. Load a page and make sure admin_menu is working
4. Applied this patch and ran the panopoly_admin.make file
5. Cleared caches
6. Reload the page

And afterward, the site gets stuck in a redirect loop. :-(

On step nr 3, everything is working fine, but to admin_menu somehow leads to the redirect loop. I skimmed over the diff between the two versions and there's nothing obviously bad in there...

Yeah, I've done and redone this a couple times and something about updating to 3.0-rc6 leads to redirect loop, whereas 3.0-rc5 doesn't.

It looks like the redirect thing is actually a bug in fieldable_panels_panes that causes this bad interaction with the latest admin_menu! Here's the FPP issue:

#3019270: Can't access to drupal 7 admin (err_too_many_redirects)

The patch is already committed but not released yet. I tried the patch and it fixed the problem in my testing. Here's a patch to panopoly_core that adds that.

If this passes automated tests, I think it's ready to commit finally!

EDIT: Here's the Travis build: https://travis-ci.org/panopoly/panopoly/builds/488747673

Tests passed! Commited :-)