Downgrade Grapht to 0.8.1 for release
Bump release to 2.1-M4
Set version back to 2.1-SNAPSNOT

This reverts commit 470383eac921bdc7e37be464d4abf3bd96c2931b.
Revert "Downgrade Grapht to 0.8.1 for release"

This reverts commit 847912c8ab07097d4deb63f820c95b05aa7a0867.