Add project metadata to the gemspec (#447)

* 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 links
will appear on the rubygems page at
https://rubygems.org/gems/cucumber-rails and be available via the
rubygems API after the next release.

* Remove wiki URI

cucumber-rails.gemspec

@@ -9,10 +9,18 @@ Gem::Specification.new do |s|
   s.description = 'Cucumber Generator and Runtime for Rails'
   s.summary     = "#{s.name}-#{s.version}"
   s.email       = 'cukes@googlegroups.com'
-  s.homepage    = 'http://cukes.info'
+  s.homepage    = 'https://cucumber.io'
 
   s.license = 'MIT'
 
+  s.metadata = {
+    'bug_tracker_uri' => 'https://github.com/cucumber/cucumber-rails/issues',
+    'changelog_uri' => "https://github.com/cucumber/cucumber-rails/blob/v#{s.version}/CHANGELOG.md",
+    'documentation_uri' => 'https://cucumber.io/docs',
+    'mailing_list_uri' => 'https://groups.google.com/forum/#!forum/cukes',
+    'source_code_uri' => "https://github.com/cucumber/cucumber-rails/tree/v#{s.version}"
+  }
+
   s.add_runtime_dependency('capybara', ['>= 2.12', '< 4'])
   s.add_runtime_dependency('cucumber', ['>= 3.0.2', '< 4'])
   s.add_runtime_dependency('mime-types', ['>= 2.0', '< 4'])