astropy_helpers not found in "cd docs; make html"

After updating to use astropy-helpers, it is no longer possible to do:
```
python setup.py develop
cd docs
make html
```
The result of this is:
```
(astropy)neptune$ make html
sphinx-build -b html -d _build/doctrees   . _build/html
Running Sphinx v1.1.3

Exception occurred:
  File "/Users/aldcroft/git/astropy/docs/conf.py", line 33, in <module>
    from astropy_helpers.sphinx.conf import *
ImportError: No module named astropy_helpers.sphinx.conf
```
One patch that fixes this case is inserting the following into `docs/conf.py`:
```
import sys
import os
sys.path.append(os.path.join('..', 'astropy_helpers'))
```