edit_on_github doesn't work out of the box

[cdeil]

During handling of the above exception, another exception occurred:

I just followed the instructions here to set up a new Astropy-affiliated package here.

I get this error from python setup.py build_sphinx if I set edit_on_github = True in setup.cfg:

Traceback (most recent call last):
  File "<stdin>", line 28, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sphinx/application.py", line 143, in __init__
    self.setup_extension(extension)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/sphinx/application.py", line 433, in setup_extension
    err)
sphinx.errors.ExtensionError: Could not import extension astropy.sphinx.ext.edit_on_github (exception: No module named 'astropy.sphinx')
Sphinx Documentation subprocess failed with return code 1

Maybe this import has to be updated to point to astropy-helpers?

[cdeil]

This is a duplicate of #119 .