[Analyzer] Only update git sourced spec repos

lib/cocoapods/installer/analyzer.rb

@@ -168,7 +168,13 @@ module Pod
       def update_repositories_if_needed
         unless config.skip_repo_update?
           UI.section 'Updating spec repositories' do
-            sources.each { |source| SourcesManager.update(source.name) if SourcesManager.git_repo?(source.repo) }
+            sources.each do |source|
+              if SourcesManager.git_repo?(source.repo)
+                SourcesManager.update(source.name)
+              else
+                UI.warn "Skipping `#{source.name}` update because the repository is not a git source repository."
+              end
+            end
           end
         end
       end