gstdec 'StructureWrapper' object has no attribute 'get_int'

Hi, and thanks for your tools !

When trying to import a flac files via beets, i got
```
beet import -m ./Estradasphere/
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/audioread/gstdec.py", line 262, in _notify_caps
self.channels = info.get_int('channels')[1]
^^^^^^^^^^^^
AttributeError: 'StructureWrapper' object has no attribute 'get_int'
```

I can search for more information if needed.

Start with audioread/gstdec.py, especially _notify_caps at the traceback line, and reproduce the beets FLAC import shown in the report. Check the StructureWrapper API available in the affected environment and verify that the import completes without this AttributeError.

Written by the indexing model from the issue text.