Fix decode error for Python 3

Hi,

`p.stdout.readlines()[0]` in `call_git_describe()` returns bytes object, which are not converted to a string object automatically in Python 3. Therefore, (for example) the string `b'v1.4.8'` will be saved to the VERSION file. After that, if the git repository is no longer available for some reason, `read_release_version()` will read the string `b'v1.4.8'` (and 'b' will be a character now!), and then `get_git_version()` will try to `decode()` it, which will lead to the error:

```
Traceback (most recent call last):
  File "version.py", line 104, in <module>
    print(get_git_version())
  File "version.py", line 100, in get_git_version
    return version[1:].decode('utf-8')
AttributeError: 'str' object has no attribute 'decode'
```

The proposed patch decodes the bytes read from the pipe, so `call_git_describe()` will return unicode string for both Python 2 and Python 3.