`yampa`: Simplify `isJustEdge`

[ivanperez-keera]

The auxiliary function FRP.Yampa.EventS.isJustEdge can be simplified by changing the order or cases:

https://github.com/ivanperez-keera/Yampa/blob/develop/yampa/src/FRP/Yampa/EventS.hs#L252-L255