Cast the border/unborder width input to CGFloat

Tl;dr: with this fix it's possible to use `border UIButton` (and probably other UIView subclasses object) without passing the layer

I was trying to use `border` on a UIButton: using `border button.layer` was working, `border button` wasn't.
So I started to debugging it: after using `border button`, printing `po button.layer` I noticed that it wasn't setting the border width.
So, in `FBDisplayCommands.py` I changed
`lldb.debugger.HandleCommand('expr (void)[%s setBorderWidth:%s]' % (layer, options.width))`
in
`lldb.debugger.HandleCommand('expr (void)[%s setBorderWidth:%s]' % (layer, 2.0))`.
A new `po button` revealed the mistake:

`<CALayer:0x8b94770; ...borderWidth = 2.8026e-45>`

Casting the `options.width` to `CGFloat` solved the issue.

Note: `unborder button` was working, but I think it's better to cast the width to `CGFloat` in any case.