Add project metadata to the gemspec (#151)

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/mini_racer and be available via the rubygems
API after the next release.

mini_racer.gemspec

@@ -14,6 +14,12 @@ Gem::Specification.new do |spec|
   spec.homepage      = "https://github.com/discourse/mini_racer"
   spec.license       = "MIT"
 
+  spec.metadata      = {
+    "bug_tracker_uri"   => "https://github.com/discourse/mini_racer/issues",
+    "changelog_uri"     => "https://github.com/discourse/mini_racer/blob/v#{spec.version}/CHANGELOG",
+    "documentation_uri" => "https://www.rubydoc.info/gems/mini_racer/#{spec.version}",
+    "source_code_uri"   => "https://github.com/discourse/mini_racer/tree/v#{spec.version}",
+  }
 
   spec.files         = `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(benchmark|test|spec|features|examples)/}) }
   spec.bindir        = "exe"