Add changelog_uri to spec metadata (#89)

This project's `NEWS.md` file appears to be the closest thing to a
changelog file that I could find. The change here links to the file in
the released version's branch. RubyGems.org will use this metadata to
display a link to this file on the gem's release pages.

Author: jgarber623

erb.gemspec

@@ -18,6 +18,7 @@ Gem::Specification.new do |spec|
 
   spec.metadata['homepage_uri'] = spec.homepage
   spec.metadata['source_code_uri'] = spec.homepage
+  spec.metadata['changelog_uri'] = "https://github.com/ruby/erb/blob/v#{spec.version}/NEWS.md"
 
   spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }