Add change log links to gem page w/o table change

This takes changelog out of the Links table

Tried to keep as much of the commits from @olivierlacan and @jaymcgavren
as possible.

app/models/links.rb

@@ -2,6 +2,7 @@ class Links
   # Links available for indexed gems
   LINKS = {
     'home'     => 'homepage_uri',
+    'changelog' => 'changelog_uri',
     'code'     => 'source_code_uri',
     'docs'     => 'documentation_uri',
     'wiki'     => 'wiki_uri',
@@ -54,10 +55,11 @@ def download_uri
 
   # define getters for each of the uris (both short `home` or long `homepage_uri` versions)
   # don't define for download_uri since it has special logic and is already defined
+  # using a try because linkset does not define all the uri attributes
   LINKS.each do |short, long|
     unless method_defined?(long)
       define_method(long) do
-        version.metadata[long].presence || linkset&.public_send(short)
+        version.metadata[long].presence || linkset.try(short)
       end
     end
     alias_method short, long

test/unit/rubygem_test.rb

@@ -457,7 +457,8 @@ class RubygemTest < ActiveSupport::TestCase
             "documentation_uri" => "http://example.com/docs",
             "mailing_list_uri" => "http://example.com/mail",
             "source_code_uri" => "http://example.com/code",
-            "bug_tracker_uri" => "http://example.com/bugs"
+            "bug_tracker_uri" => "http://example.com/bugs",
+            "changelog_uri" => "http://example.com/change"
           }
         )
 
@@ -469,6 +470,7 @@ class RubygemTest < ActiveSupport::TestCase
         assert_equal "http://example.com/mail", hash["mailing_list_uri"]
         assert_equal "http://example.com/code", hash["source_code_uri"]
         assert_equal "http://example.com/bugs", hash["bug_tracker_uri"]
+        assert_equal "http://example.com/change", hash["changelog_uri"]
       end
 
       should "return version documentation url if metadata and linkset docs is empty" do