improve version configuration code
improve version configuration code

* fixes #35

neurord/pysb,d-fan/pysb,johnbachman/pysb,spgarbet/pysb,spgarbet/pysb,alubbock/pysb-legacy,LoLab-VU/pysb,pysb/pysb,alubbock/pysb,jmuhlich/pysb,alubbock/pysb-legacy,spgarbet/pysb,neurord/pysb,d-fan/pysb,neurord/pysb,LoLab-VU/pysb,d-fan/pysb,alubbock/pysb-legacy,alubbock/pysb-legacy,johnbachman/pysb