Add compatibility with Nova 2.1.0

[bmichotte]

Nova 2.1.0 added showOnIndex($callback) on Laravel\Nova\Fields\FieldElement

This PR add the compatibility with it to avoid the following error

Declaration of Froala\NovaFroalaField\Froala::showOnIndex() should be compatible with Laravel\Nova\Fields\FieldElement::showOnIndex($callback = true)

[niekdemelker]

Why not remove the function here? you don't really need the overwrite and its more future proof. Don't overwrite what doesn't need overwriting. Overwriting is what caused this issue in the first place.

[bmichotte]

@niekdemelker, I’m totally agree with you, but as the original code was forcing showOnIndex to true, I kept the same behavior.

But we are agree that this method should be removed

[slavarazum]

Method removed - #32