Update cucumber distributed formatter

As discussed in #59 this change allows to support cucumber version >=1.2.2

In https://github.com/cucumber/cucumber/commit/f74a8dad3e8ff1b1e9fe7762369266817f229a8f
`step_mother` was renamed to `runtime`. This fix allows to work with cucumber versions >=1.2.2 without updating the specjour gem dependency requirements.


Note: cucumber version 1.3 is still not supported.