Fixes an issue with the build_sphinx test where it wasn't restoring the cwd correctly, and was also copying more of astropy_helpers than was necessary.
Fix a mostly unrelated issue with build_sphinx that seems to be affecting the Travis builds.  Don't be as strict about where the 'build succeeded' message appears in the Sphinx output.  (Why do we even do this? Does sphinx not produce a usable return code?)
Merge pull request #111 from embray/fix-build-sphinx-test

Fixes an issue with the build_sphinx test