Fix logic in Project.conf_dir
Fix logic in Project.conf_dir

The original implementation replaced "/conf.py" with an empty string, where it
only wanted to return the directory name of the conf.py file. This resulted in
problems when "/conf.py" was part of the directory name!

Fixes #1553.

tddv/readthedocs.org,espdev/readthedocs.org,emawind84/readthedocs.org,davidfischer/readthedocs.org,emawind84/readthedocs.org,emawind84/readthedocs.org,pombredanne/readthedocs.org,davidfischer/readthedocs.org,istresearch/readthedocs.org,stevepiercy/readthedocs.org,emawind84/readthedocs.org,clarkperkins/readthedocs.org,stevepiercy/readthedocs.org,wijerasa/readthedocs.org,rtfd/readthedocs.org,gjtorikian/readthedocs.org,stevepiercy/readthedocs.org,pombredanne/readthedocs.org,rtfd/readthedocs.org,istresearch/readthedocs.org,espdev/readthedocs.org,safwanrahman/readthedocs.org,gjtorikian/readthedocs.org,espdev/readthedocs.org,SteveViss/readthedocs.org,tddv/readthedocs.org,espdev/readthedocs.org,espdev/readthedocs.org,techtonik/readthedocs.org,gjtorikian/readthedocs.org,rtfd/readthedocs.org,tddv/readthedocs.org,wijerasa/readthedocs.org,istresearch/readthedocs.org,clarkperkins/readthedocs.org,clarkperkins/readthedocs.org,davidfischer/readthedocs.org,pombredanne/readthedocs.org,istresearch/readthedocs.org,safwanrahman/readthedocs.org,wijerasa/readthedocs.org,SteveViss/readthedocs.org,rtfd/readthedocs.org,techtonik/readthedocs.org,stevepiercy/readthedocs.org,davidfischer/readthedocs.org,wijerasa/readthedocs.org,safwanrahman/readthedocs.org,safwanrahman/readthedocs.org,SteveViss/readthedocs.org,gjtorikian/readthedocs.org,clarkperkins/readthedocs.org,techtonik/readthedocs.org,techtonik/readthedocs.org,SteveViss/readthedocs.org