I switched my buildout from Plone 5.0 to Plone 5.1
[buildout] extends = - https://raw.github.com/collective/buildout.plonetest/master/test-5.x.cfg + https://raw.github.com/collective/buildout.plonetest/master/test-5.1.x.cfg base.cfg
However this fails with
bin/buildout -c dev5.cfg While: Initializing. Error: Error downloading extends for URL http://dist.plone.org/release/5.1a1-pending/: HTTP Error 404: Not Found
What is the proper way for testing Plone 5.1a1 right now?