Update promulgated ubuntu-repository-cache charm


The currently promulgated ubuntu-repository-cache charm points at one that seems no longer maintained:

$ charm show cs:ubuntu-repository-cache
Ubuntu archive caching proxy mirror

Name                    ubuntu-repository-cache
Owner                   cloud-images
Revision                24
Supported Series        xenial, trusty
Tags                    cache-proxy

Can we please update it to cs:~ubuntu-repository-cache-charmers/ubuntu-repository-cache?



@kwmonroe, @rick_h, any of you able to assist with this?

Found a request I filed a year ago about this too heh.