The master is the new next_stable

[tvdeyen]

We recently changed our branch workflow.

1. The master is the new development branch for the next stable release version (2.2 as time of writing).
2. The current stable releases has their own branches (2.1-stable for current stable version 2.1.7).
3. Use these branches for patch releases (2.1.x).

We removed the next_stable branch.