Conditionally include `sudo` in suggested update command

Closes https://github.com/CocoaPods/CocoaPods/issues/2058

lib/cocoapods/sources_manager.rb

@@ -205,9 +205,12 @@ module Pod
           "Update CocoaPods, or checkout the appropriate tag in the repo."
         end
 
+        needs_sudo = path_writable?(__FILE__)
+
         if config.new_version_message? && cocoapods_update?(versions)
           UI.puts "\nCocoaPods #{versions['last']} is available.\n" \
-            "To update use: [sudo] gem install cocoapods\n".green
+            "To update use: #{needs_sudo ? 'sudo ' : ''}" \
+            'gem install cocoapods\n'.green
         end
       end
 
@@ -284,6 +287,12 @@ module Pod
 
       #-----------------------------------------------------------------------#
 
+      private
+
+      def path_writable?(path)
+        Pathname(path).dirname.writable?
+      end
+
     end
   end
 end

spec/unit/sources_manager_spec.rb

@@ -146,6 +146,12 @@ module Pod
         e.message.should.match /Update CocoaPods/
       end
 
+      it 'returns whether a path is writable' do
+        path = '/Users/'
+        Pathname.any_instance.stubs(:writable?).returns(true)
+        SourcesManager.send(:path_writable?, path).should.be.true
+      end
+
       it "returns whether a repository is compatible" do
         SourcesManager.stubs(:version_information).returns({ 'min' => '0.0.1' })
         SourcesManager.repo_compatible?('stub').should.be.true