Add project metadata to the gemspec (#168)

* Use TLS in homepage URI
* 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` and
`source_code_uri` links will appear on the rubygems page at
https://rubygems.org/gems/redis-namespace and be available via the
Rubygems API after the next release.

redis-namespace.gemspec

@@ -7,11 +7,18 @@ Gem::Specification.new do |s|
   s.version           = Redis::Namespace::VERSION
   s.date              = Time.now.strftime('%Y-%m-%d')
   s.summary           = "Namespaces Redis commands."
-  s.homepage          = "http://github.com/resque/redis-namespace"
+  s.homepage          = "https://github.com/resque/redis-namespace"
   s.email             = ["chris@ozmm.org", "hone02@gmail.com", "steve@steveklabnik.com", "me@yaauie.com"]
   s.authors           = ["Chris Wanstrath", "Terence Lee", "Steve Klabnik", "Ryan Biesemeyer"]
   s.license           = 'MIT'
 
+  s.metadata = {
+    "bug_tracker_uri"   => "https://github.com/resque/redis-namespace/issues",
+    "changelog_uri"     => "https://github.com/resque/redis-namespace/blob/master/CHANGELOG.md",
+    "documentation_uri" => "https://www.rubydoc.info/gems/redis-namespace/#{s.version}",
+    "source_code_uri"   => "https://github.com/resque/redis-namespace/tree/v#{s.version}",
+  }
+
   s.files             = %w( README.md Rakefile LICENSE )
   s.files            += Dir.glob("lib/**/*")
   s.files            += Dir.glob("test/**/*")