Don't throw an error when --deb-compression=gz is passed.

Previously, when passing `--deb-compression=gz`, you'd get

```
Invalid package configuration: Unknown compression type 'gz' {:level=>:error}
```

And when passing `--deb-compression=gzip`, you'd get

```
ERROR: option '--deb-compression': deb compression value of 'gzip' is invalid. Must be one of gz, bzip2, xz
```

This makes `--deb-compression=gz` work happily.