Update astropy_helpers sub-module pointer to latest master

I think this is needed to work around the following error with Sphinx 1.2.3:
```
$ python setup.py build_sphinx
Traceback (most recent call last):
  File "<stdin>", line 27, in <module>
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sphinx/application.py", line 147, in __init__
    self._init_builder(buildername)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sphinx/application.py", line 209, in _init_builder
    self.emit('builder-inited')
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sphinx/application.py", line 408, in emit
    results.append(callback(self, *args))
  File "/Users/deil/code/gammapy/astropy_helpers/astropy_helpers/sphinx/ext/automodsumm.py", line 234, in process_automodsumm_generation
    base_path=app.srcdir)
  File "/Users/deil/code/gammapy/astropy_helpers/astropy_helpers/sphinx/ext/automodsumm.py", line 419, in generate_automodsumm_docs
    name, obj, parent = import_by_name(name)
ValueError: too many values to unpack
```