When running `dep prune` explicitly, the following message show up:
```
dep prune
Pruning is now performed automatically by dep ensure.
```
However, after the explicit `dep prune`, there are still many files deleted. (Guess `dep ensure` is not complete yet).
This fix did a `dep prune` to clean up unneeded files.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>
c7321fabc2