Make compatible with Cucumber 1.2.3's rename of step_mother to runtime

See cucumber/cucumber@f74a8dad3e8ff1b1e9fe7762369266817f229a8f

Also maintains backwards compatibility with previous Cucumber versions
by aliasing step_mother to runtime.
Merge pull request #23 from amarshall/rename-step_method-to-runtime

Make compatible with Cucumber 1.2.3's rename of step_mother to runtime