Configuration warning on stable versions when running test suite

[astrofrog]

I noticed the following issue on Travis for v4.0.x - the Python 3.6 with oldest supported version of all dependencies build fails when pushing the commit that contains a stable (non dev) version:

https://travis-ci.org/github/astropy/astropy/jobs/734636060

The warning, made into an error, is:

ImportError while loading conftest '/home/travis/build/astropy/astropy/conftest.py'.
conftest.py:18: in <module>
    import astropy
astropy/__init__.py:371: in <module>
    _initialize_astropy()
astropy/__init__.py:298: in _initialize_astropy
    config.configuration.update_default_config(__package__, config_dir)
astropy/config/configuration.py:738: in update_default_config
    ConfigurationChangedWarning)
E   astropy.config.configuration.ConfigurationChangedWarning: The configuration options in astropy 4.0.2 may have changed, your configuration file was not updated in order to preserve local changes.  A new configuration template has been saved to '/home/travis/.astropy/config/astropy.4.0.2.cfg'.

I never noticed this because often that commit is pushed once the release branch CI is otherwise passing. I tried clearing the Travis cache to no avail.

[pllim]

We don't use Travis no more.