fixing gzip import issue 2774

Changed import from:
import gzip
to:
from ...utils.compat import gzip

ran setup.py test and all io/fits tests passed. I don't have Python 2.6 to test that this really fixed the problem, but it should based on the comments in issue #2774
Sorry, one last thing--could you fix this to read something more specific to how the original problem manifested?  Something like:

```
Fixed crash when reading gzip-compressed FITS tables through the Astropy
``Table`` interface. [#2783]
```

Then also rebase your branch on `upstream/master` and delete the merge commits from the history.  See http://docs.astropy.org/en/stable/development/workflow/additional_git_topics.html?highlight=rebasing#rebasing-on-trunk  (it talks about "trunk" for people coming from SVN, but it should probably say "master" instead of "trunk"--I'll see about rewriting that at some point).