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 rubygems/rubygems.org#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.

rubocop.gemspec

@@ -4,6 +4,7 @@ $LOAD_PATH.unshift File.expand_path('../lib', __FILE__)
 require 'rubocop/version'
 require 'English'
 
+# rubocop:disable Metrics/BlockLength
 Gem::Specification.new do |s|
   s.name = 'rubocop'
   s.version = RuboCop::Version::STRING
@@ -21,10 +22,20 @@ Gem::Specification.new do |s|
             .split($RS)
   s.executables = s.files.grep(%r{^bin/}) { |f| File.basename(f) }
   s.extra_rdoc_files = ['LICENSE.txt', 'README.md']
-  s.homepage = 'http://github.com/bbatsov/rubocop'
+  s.homepage = 'https://github.com/bbatsov/rubocop'
   s.licenses = ['MIT']
   s.summary = 'Automatic Ruby code style checking tool.'
 
+  if s.respond_to?(:metadata=)
+    s.metadata = {
+      'homepage_uri' => 'https://rubocop.readthedocs.io/',
+      'changelog_uri' => 'https://github.com/bbatsov/rubocop/blob/master/CHANGELOG.md',
+      'source_code_uri' => 'https://github.com/bbatsov/rubocop/',
+      'documentation_uri' => 'https://rubocop.readthedocs.io/',
+      'bug_tracker_uri' => 'https://github.com/bbatsov/rubocop/issues'
+    }
+  end
+
   s.add_runtime_dependency('rainbow', '>= 2.2.2', '< 3.0')
   s.add_runtime_dependency('parser', '>= 2.3.3.1', '< 3.0')
   s.add_runtime_dependency('powerpack', '~> 0.1')
@@ -34,3 +45,4 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency('bundler', '~> 1.3')
 end
+# rubocop:enable Metrics/BlockLength