Remove version from cpio dependency

Now that the `target` repository only ships an earlier, GPL-3-free version
of `cpio` provided by `cpio-gplv2`, the versioned dependency fails:

```
2019/09/11 16:28:32 apt | The following packages have unmet dependencies:
2019/09/11 16:28:32 apt |  initramfs-tools : Depends: initramfs-tools-core (= 0.132co1ba1) but it is not going to be installed
2019/09/11 16:28:32 apt | E: Unable to correct problems, you have held broken packages.
```

The versioned depends was introduced for the `--reproducible` flag, used if
`SOURCE_DATE_EPOCH` is set (which we don't use):
https://salsa.debian.org/kernel-team/initramfs-tools/commit/f1cbe6f18c925cffd85



Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>