Fix invalid argument order (#832)

Make sure that --dev purge-chain is rejected

```
[0] [11:24:02] ~/r/polkadot master > ./target/debug/polkadot purge-chain --dev -y
"/home/cecile/.local/share/polkadot/chains/dev/db" did not exist.
[0] [11:25:36] ~/r/polkadot cecton-fix-invalid-argument-order > ./target/debug/polkadot --dev purge-chain -y
error: Found argument 'purge-chain' which wasn't expected, or isn't valid in this context

USAGE:
    polkadot --dev

For more information try --help
```