I edited a couple of pinned versions in buildout.cfg to update my Plone 5.1 site, including setting plone.app.mosaic==2.1.0.
Upon re-running buildout I got the message:
... Error: The requirement ('plone.app.mosaic==2.1.0') is not allowed by your [versions] constraint (2.0rc8)
version 2.1.0 is very new.
The 5.1/version.cfg should stay as it was so no working Plone site gets updated without one knowing it (while running buildout).
For yourself, you can just edit change mosaic version in versions.cfg by downloading the file, adding it in buildout.cfg instead of the dist.org