Sort results of `os.listdir` in step discovery in runner.py to fix upstream issue #122

This fixes [Issue #122 on behave/behave](https://github.com/behave/behave/issues/122) (the issue is with this repository, jenisys/behave, but issue tracking is not enabled for this repository). See that thread for details, but in short, there were inconsistencies between expected discovery order of step files and actual discovery order.