make the dependency to brdr- version less specific

[dieuska]

at the moment a specific version of brdr is mandatory (brdr==0.1.0).

This has to be made less specific (brdr>=0.1.0) on release so minor releases can also be used, but still important to check the backwards compatibility

[dieuska]

At the moment, the versioning of brdr has to stabilize before executing this. So we put this 'on hold'