Don't check for full package in issue 112

Pypi doesn't output package for AmbigousStep, which breaks [pypy self-check suite on travis](https://s3.amazonaws.com/archive.travis-ci.org/jobs/5779205/log.txt):
```
Expected: a string containing u'behave.step_registry.AmbiguousStep:[snip]
     but: was '[snip]\nAmbiguousStep: "I buy {number:n} {items:w}" has already been defined in\nexisting step: "I buy {amount} {product}" (features/steps/bad_steps.py:4)'

```