Update `grunt-contrib-uglify`.

Gruntfile.js

@@ -26,7 +26,7 @@ module.exports = function(grunt) {
 		uglify: {
 			options: {
 				banner: '<%= meta.banner %>\n',
-				screwIE8: false
+				ie8: true
 			},
 			build: {
 				src: 'js/reveal.js',

package.json

@@ -31,7 +31,7 @@
     "grunt-contrib-cssmin": "^2.2.1",
     "grunt-contrib-jshint": "^1.1.0",
     "grunt-contrib-qunit": "~1.2.0",
-    "grunt-contrib-uglify": "^2.3.0",
+    "grunt-contrib-uglify": "^3.3.0",
     "grunt-contrib-watch": "^1.0.0",
     "grunt-sass": "^2.0.0",
     "grunt-retire": "^1.0.7",