[Analyzer] Only update source repos that are being used

lib/cocoapods/installer/analyzer.rb

@@ -169,7 +169,7 @@ module Pod
       def update_repositories_if_needed
         unless config.skip_repo_update?
           UI.section 'Updating spec repositories' do
-            SourcesManager.update
+            sources.each { |source| SourcesManager.update(source.name) }
           end
         end
       end

spec/unit/installer/analyzer_spec.rb

@@ -62,7 +62,14 @@ module Pod
 
       it 'updates the repositories by default' do
         config.skip_repo_update = false
-        SourcesManager.expects(:update).once
+        SourcesManager.expects(:update).twice
+        @analyzer.analyze
+      end
+
+      it 'does not update unused sources' do
+        config.skip_repo_update = false
+        @analyzer.stubs(:sources).returns(SourcesManager.master)
+        SourcesManager.expects(:update).once.with('master')
         @analyzer.analyze
       end