Make versioning more robust

@elisachisari reported some issues with installation due to the way that we are setting the version number in [LSSTDESC/CCL#121](https://github.com/LSSTDESC/CCL/pull/121#issuecomment-286455367). This is probably related to the messy code for backward compatibility [here](https://github.com/DarkEnergySurvey/mkauthlist/blob/fe5c53b16313a62a187b1af60519537a76193fb7/mkauthlist/mkauthlist.py#L23-L32).