Show version only once with '-v' option in mruby/mrbc

## current
```bash
$ mruby -v -v -v
mruby - Embeddable Ruby  Copyright (c) 2010-2013 mruby developers
mruby - Embeddable Ruby  Copyright (c) 2010-2013 mruby developers
mruby - Embeddable Ruby  Copyright (c) 2010-2013 mruby developers

```

## fixed
```bash
$ mruby -v -v -v
mruby - Embeddable Ruby  Copyright (c) 2010-2013 mruby developers
```