IMP stable branch is now 'main' not 'master'

Author: benmwebb

README.md

@@ -72,7 +72,7 @@ _License_: [CC-BY-SA-4.0](https://creativecommons.org/licenses/by-sa/4.0/legalco
 This work is freely available under the terms of the Creative Commons
 Attribution-ShareAlike 4.0 International License.
 
-_Last known good IMP version_: [![build info](https://integrativemodeling.org/systems/20/badge.svg?branch=master)](https://integrativemodeling.org/systems/) [![build info](https://integrativemodeling.org/systems/20/badge.svg?branch=develop)](https://integrativemodeling.org/systems/)
+_Last known good IMP version_: [![build info](https://integrativemodeling.org/systems/20/badge.svg?branch=main)](https://integrativemodeling.org/systems/) [![build info](https://integrativemodeling.org/systems/20/badge.svg?branch=develop)](https://integrativemodeling.org/systems/)
 
 _Publications_:
  - Chen ZA, Pellarin R, Fischer L, Sali A, Nilges M, Barlow PN, Rappsilber J.,