Added prerequisites up to Rotational Cipher (#2080)
Added prerequisites up to Rotational Cipher (#2080)

* Added prerequisites up to Rotational Cipher

I started to add the prerequisites for the exercises. I will follow the order the exercises appear in the config.json file.

I also updated the prerequisites of the connect exercise, to match the concepts in the concepts array.

* Reviewed prerequisites
jmluy/xjava,jmluy/xjava