Add linkchecker to build_sphinx test on travis-ci

Sphinx doesn't warn on a few cases with broken HTML internal links in the docs (not sure why).

These are fixed here and `linkchecker` is run on travis-ci (as e.g. [matplotlib does as well](https://github.com/matplotlib/matplotlib/blob/master/.travis.yml)), for internal links only so that will *not* cause any test failures if external webpages are down.
The reason I added `--ignore-url astropy/wcs/_wcs.html` is that without it there's this broken link:
```
URL        `astropy/wcs/_wcs.html'
Name       `astropy.wcs._wcs'
Parent URL file:///Users/deil/code/astropy/docs/_build/html/_modules/index.html, line 202, col 9
Real URL   file:///Users/deil/code/astropy/docs/_build/html/_modules/astropy/wcs/_wcs.html
Check time 0.001 seconds
Result     Error: URLError: <urlopen error [Errno 2] No such file or directory: '/Users/deil/code/astropy/docs/_build/html/_modules/astropy/wcs/_wcs.html'>
```

The [_modules/index.html](http://astropy.readthedocs.org/en/latest/_modules/index.html) page currently contains a link to the non-existing [modules/astropy/wcs/_wcs.html](http://astropy.readthedocs.org/en/latest/_modules/astropy/wcs/_wcs.html) page.
I think the `_modules/index.html` is not reachable from the docs in the first place, so it's OK to ignore this problem.
But @embray or @mdboom, if you know how to fix this in the Sphinx build, we could remove `--ignore-url astropy/wcs/_wcs.html` here.