Test failure on Debian/Hurd

When building version 0.2.1 on Debian/Hurd-i386, I get a some failures due to exceptions:

```
astropy/io/fits/tests/test_checksum.py:200: TestChecksumFunctions.test_open_update_mode_update_checksum FAILED
astropy/io/fits/tests/test_hdulist.py:634: TestHDUListFunctions.test_save_backup FAILED
astropy/io/fits/tests/test_image.py:113: TestImageFunctions.test_io_manipulation FAILED
astropy/io/fits/tests/test_image.py:760: TestImageFunctions.test_image_update_header FAILED
astropy/io/fits/tests/test_image.py:779: TestImageFunctions.test_open_scaled_in_update_mode FAILED
astropy/io/fits/tests/test_table.py:1908: TestTableFunctions.test_bool_column_update FAILED
```

They burn down to
```
astropy/io/fits/hdu/hdulist.py:913: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <astropy.io.fits.hdu.table.BinTableHDU object at 0x9e48f6c>
fileobj = <astropy.io.fits.file._File <open file '/tmp/fits-test-orFqme/table.fits', mode 'rb+' at 0x9b54230>>
inplace = True, copy = False

    def _writeto(self, fileobj, inplace=False, copy=False):
[...]
        if self._data_loaded:
[...]
                if array_mmap is not None:
>                   array_mmap.flush()
E                   error: [Errno 1073741902] Function not implemented

astropy/io/fits/hdu/base.py:540: error
```
Full log is in
https://buildd.debian.org/status/fetch.php?pkg=python-astropy&arch=hurd-i386&ver=0.2.1-1%7Eexp2&stamp=1365631121