Revoke command parameter -aki is case sensitive

Using sqlite and the `revoke` command's `-aki` parameter, it appears the database lookup is case sensitive.

```bash
$ cfssl certinfo -cert client-server-name.pem
...
"serial_number": "592549817469832368177901454950774200973085431858",
...
"authority_key_id": "44:F1:61:22:A7:49:79:36:38:F7:13:46:3E:88:B1:FA:01:44:21:A1",
```

Failure:

```bash
$ cfssl revoke -db-config db-config.json -serial 592549817469832368177901454950774200973085431858 -aki 44F16122A749793638F713463E88B1FA014421A1 -reason superseded
{"code":11200,"message":"failed to revoke the certificate: certificate not found"}
```

Success:

```bash
$ cfssl revoke -db-config db-config.json -serial 592549817469832368177901454950774200973085431858 -aki 44f16122a749793638f713463e88b1fa014421a1 -reason superseded
```