Remove caching of ratio to allow change of image src

I just ran into an issue with using this plugin in an application where the src attribute is switched out dynamically and I wanted to re-object-fit 'contain' the same image tag. I worked around it by `$('img').removeData('ratio')` but when I started looking through this source, I had a hard time understanding the need for caching the ratio.

All of the information to perform the calculation is available at the time of checking the cache and it's just a simple division away. Please let me know your thoughts. 

BTW. Thanks so much for all of your work on this plugin!