brew-create: don't break when given --set-version

Since 9f2782812cbaf2, specifying a version breaks with:

    Error: undefined method `detected_from_url?' for "1.2.2":String

All this logic being performed in the `url=` setter method, order-dependent on the calls above,

```ruby
    fc = FormulaCreator.new
    fc.name = name
    fc.version = version
    fc.url = url
```

is pretty gross. I'll do some refactoring if you'd like, but wanted to keep this fix to the point.