change remote data tests to use astropy site instead of google

This issue came up again in #2405 (although it has been around since at least #491).

The basic issue is that in some of the remote data tests, we compare to google pages.  But google gives different encodings depending on what country you're in.  This just sidesteps the problem by instead getting the Astropy web site (which should always be UTF-8, unless we change it in the future).  That's fairly safe in terms of travis tests, because it's hosted on github, and if github is down, probably travis won't be getting updated, anyway.

cc @astrofrog @embray