Update the call to get_package_info since its interface changed
Have the doctest plus module skip/ignore any modules that raise an ImportError when trying to import it to look for tests.  In particular this is needed to skip modules that require astropy_helpers, like some of the setup_package.py modules.  This seems like reasonable-enough behavior anyway--if some actual problem is causing ImportErrors in some modules that will hopefully be picked up by the normal unit tests as well.
Update to latest astropy_helpers; now this will use the test command directly from astropy_helpers.