Replace deprecated FastGFile with GFile

FastGFile has been deprecated and replaced with GFile, though
the example in speech_commands still uses FastGFile. This fix
fix the issue to remove the deprecated warning:
```
WARNING:tensorflow:From <stdin>:1: __init__ (from tensorflow.python.platform.gfile) is deprecated and will be removed in a future version.
Instructions for updating:
Use tf.gfile.GFile.
```

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>