Check rippleImage existence before making it

The implementation of ```rippleImage``` property should check whether the ```_rippleImage``` is nil or not. (Just like ```touchImage```)

Current implementation just set the ivar directly which makes 
1. Custom image will be replaced when calling this property. (which is not what we want :-1: )
2. The default image is redrawn every time calling this property. (it's inefficiency)