Add project metadata to the gemspec

As per https://guides.rubygems.org/specification-reference/#metadata,
add metadata to the gemspec file. This'll allow people to more easily
access the source code, raise issues and read the changelog. These
`bug_tracker_uri`, `changelog_uri`, `documentation_uri`, `wiki_uri` and
`source_code_uri` links will appear on the rubygems page at
https://rubygems.org/gems/multi_json and be available via the rubygems
API after the next release.

multi_json.gemspec

@@ -7,13 +7,21 @@ Gem::Specification.new do |spec|
   spec.summary       = "A common interface to multiple JSON libraries."
   spec.description   = "A common interface to multiple JSON libraries, including Oj, Yajl, the JSON gem (with C-extensions), the pure-Ruby JSON gem, NSJSONSerialization, gson.rb, JrJackson, and OkJson."
   spec.files         = Dir["*.md", "lib/**/*"]
-  spec.homepage      = "http://github.com/intridea/multi_json"
+  spec.homepage      = "https://github.com/intridea/multi_json"
   spec.license       = "MIT"
   spec.name          = "multi_json"
   spec.require_path  = "lib"
   spec.version       = MultiJson::Version
 
+  spec.metadata = {
+    'bug_tracker_uri'   => 'https://github.com/intridea/multi_json/issues',
+    'changelog_uri'     => "https://github.com/intridea/multi_json/blob/v#{spec.version}/CHANGELOG.md",
+    'documentation_uri' => "https://www.rubydoc.info/gems/multi_json/#{spec.version}",
+    'source_code_uri'   => "https://github.com/intridea/multi_json/tree/v#{spec.version}",
+    'wiki_uri'          => 'https://github.com/intridea/multi_json/wiki'
+  }
+
   spec.required_rubygems_version = ">= 1.3.5"
   spec.add_development_dependency "rake", "~> 10.5"
   spec.add_development_dependency "rspec", "~> 3.9"
-end
+end