External Libraries: Update Backbone to version 1.3.3.

Changelog: https://cdn.rawgit.com/jashkenas/backbone/1.3.3/index.html#changelog
Diff: https://github.com/jashkenas/backbone/compare/1.2.3...1.3.3

Includes a unit test to ensure that the minified version doesn't include `sourceMappingURL`.

Props adamsilverstein.
Fixes #37099.

git-svn-id: https://develop.svn.wordpress.org/trunk@37723 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/dependencies/backbonejs.php

@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * @group dependencies
+ * @group scripts
+ */
+class Tests_Dependencies_Backbonejs extends WP_UnitTestCase {
+
+	function test_exclusion_of_sourcemaps() {
+		$file = ABSPATH . WPINC . '/js/backbone.min.js';
+		$this->assertTrue( file_exists( $file ) );
+		$contents = trim( file_get_contents( $file ) );
+		$this->assertFalse( strpos( $contents, 'sourceMappingURL' ), 'Presence of sourceMappingURL' );
+	}
+}