audio/m4a files are matched as video/mp4

To reproduce:

```python
In [1]: with open("foo.m4a", 'rb') as f:
   ...:     data = f.read()
   ...:
In [2]: import filetype
In [3]: filetype.match(data)
Out[3]: <filetype.types.video.Mp4 at 0x7fbab95fb160>
```