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/rake and be available via the Rubygems API
after the next release.

Author: orien

rake.gemspec

@@ -23,6 +23,13 @@ Rake has the following features:
   s.homepage = "https://github.com/ruby/rake".freeze
   s.licenses = ["MIT".freeze]
 
+  s.metadata = {
+    "bug_tracker_uri"   => "https://github.com/ruby/rake/issues",
+    "changelog_uri"     => "https://github.com/ruby/rake/blob/v#{s.version}/History.rdoc",
+    "documentation_uri" => "https://ruby.github.io/rake",
+    "source_code_uri"   => "https://github.com/ruby/rake/tree/v#{s.version}",
+  }
+
   s.files = %x[git ls-files -z].split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) } -
             %w[.rubocop.yml .gitignore .travis.yml appveyor.yml]
   s.bindir = "exe"