Always depend on gnureadline in setup.py if setupext.check_for_readline() fails

The asymmetry (inconsistency) of unconditionally adding `gnureadline` as a dependency of binary (but not source) distributions in `setup.py` can cause issues like reported at paylogic/pip-accel/issues/34. Would you be willing to accept this pull request to remove the asymmetry? For reference, it seems that this was introduced in ba0cd52c with the following comment:

> adjust dependency check for gnureadline on OS X
> 
> - always depend when making a bdist (for consistency)
> - otherwise only depend if missing (e.g. pip install -e)

I guess it's kind of ironic that the original reasoning was consistency and now I'm calling this inconsistent :-). I'm interested to hear whether you agree about the asymmetry though. If you need more context about why I want to fix the asymmetry please refer to paylogic/pip-accel/issues/34, I'm posting a detailed explanation there.