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, homepage_uri and
source_code_uri links will appear on the rubygems page at
https://rubygems.org/gems/redis and be available via the rubygems API
after the next release.

redis.gemspec

@@ -32,6 +32,14 @@ Gem::Specification.new do |s|
 
   s.email = ["redis-db@googlegroups.com"]
 
+  s.metadata = {
+    "bug_tracker_uri" => "#{s.homepage}/issues",
+    "changelog_uri" => "#{s.homepage}/blob/master/CHANGELOG.md",
+    "documentation_uri" => "https://www.rubydoc.info/gems/redis/#{s.version}",
+    "homepage_uri" => s.homepage,
+    "source_code_uri" => "#{s.homepage}/tree/v#{s.version}"
+  }
+
   s.files         = Dir["CHANGELOG.md", "LICENSE", "README.md", "lib/**/*"]
   s.executables   = `git ls-files -- exe/*`.split("\n").map { |f| File.basename(f) }