doctest in sphinx documentation are now failing

Following commit https://github.com/astropy/astropy/commit/e6d19c33bd8fc861e8660a58519b1d26349a69cb, sphinx documentations started being tested, causing failures in e.g. https://travis-ci.org/astropy/astroquery/builds/14483606.

Almost all of astroquery examples will fail due to turned off internet access.

Should we, per http://docs.astropy.org/en/latest/development/testguide.html#skipping-doctests, add to `./docs/*.rst` this command `.. doctest-skip-all`?