Add gemspec metadata
Add gemspec metadata

Makes it easy to programatically find the source code and changelog for rubocop,
using the rubygems API. Also adds a changelog link etc. to
https://rubygems.org/gems/rubocop

See https://github.com/rubygems/rubygems.org/pull/1234 and
https://github.com/rubygems/rubygems.org/blob/master/app/models/links.rb for
more details.

I did not add `wiki_uri`, `mailing_list_uri` and `download_uri` because 1) the
only wiki I could find (https://github.com/bbatsov/rubocop/wiki) looks
abandoned; 2) there is no mailing list that I know of, and 3) rubygems.org will
add a download link, e.g. https://rubygems.org/downloads/rubocop-0.49.1.gem if
we don't add one.

Also changing `s.homepage` to use https protocol.

jmks/rubocop,tejasbubane/rubocop,maxjacobson/rubocop,panthomakos/rubocop,petehamilton/rubocop,mikegee/rubocop,alexcstark/rubocop,jfelchner/rubocop,tejasbubane/rubocop,tdeo/rubocop,meganemura/rubocop,bquorning/rubocop,panthomakos/rubocop,haziqhafizuddin/rubocop,smakagon/rubocop,panthomakos/rubocop,palkan/rubocop,petehamilton/rubocop,scottmatthewman/rubocop,palkan/rubocop,akihiro17/rubocop,bquorning/rubocop,jmks/rubocop,mikegee/rubocop,rrosenblum/rubocop,petehamilton/rubocop,sue445/rubocop,jfelchner/rubocop,palkan/rubocop,rrosenblum/rubocop,bquorning/rubocop,deivid-rodriguez/rubocop,jmks/rubocop,scottmatthewman/rubocop,deivid-rodriguez/rubocop,haziqhafizuddin/rubocop,tdeo/rubocop,bbatsov/rubocop,tejasbubane/rubocop,vergenzt/rubocop,akihiro17/rubocop,deivid-rodriguez/rubocop,maxjacobson/rubocop,meganemura/rubocop,urbanautomaton/rubocop,urbanautomaton/rubocop,sue445/rubocop,jfelchner/rubocop,bbatsov/rubocop,maxjacobson/rubocop,akihiro17/rubocop,smakagon/rubocop,tdeo/rubocop,vergenzt/rubocop,mikegee/rubocop,alexcstark/rubocop,pocke/rubocop,pocke/rubocop,rrosenblum/rubocop,b-t-g/rubocop,meganemura/rubocop,vergenzt/rubocop,b-t-g/rubocop,sue445/rubocop