Add recipe for astropy 0.4 and script to automate builds

There was one issue of note that came up in trying to build the conda packages for astropy 0.4: ``conda build`` disallows downloading by setuptools, which raises an error when ``ah_bootstrap.py`` tries to check for a new version of ``astropy_helpers``. 

I fixed it for now by adding ``--offline`` to ``setup.py install`` in ``build.sh``; I suppose a better solution would be building a conda package for ``astropy_helpers``, keeping that up to date, and using that package.