title: Grunt uglifying step failing
body: <p>I am new to Grunt and I am trying to use it with this config </p>
  
  <pre><code>module.exports = function(grunt) {
  
    grunt.initConfig({
      pkg: grunt.file.readJSON('package.json'),
  
      // Define our source and build folders
      js_src_path: 'javascripts',
      js_build_path: "build/js",
  
  // Grunt Tasks
      concat: {
        options:{
          separator: ';'
        },
        js: {
          src: ['&lt;%= js_src_path %&gt;/*.js'],
          dest: '&lt;%= js_build_path %&gt;/app.js'
        }    },
      uglify: {
        options: {
          banner: '/*! &lt;%= pkg.name %&gt; &lt;%= grunt.template.today("yyyy-mm-dd") %&gt; */\n'
        },
        js: {
          src: '&lt;%= concat.js.dest %&gt;',
          dest:'&lt;%= js_build_path %&gt;/app.min.js'
        }
      }
      }
    );
  
    grunt.loadNpmTasks('grunt-contrib-concat');
    grunt.loadNpmTasks('grunt-contrib-uglify');
  
    // Default task.
    grunt.registerTask('default', ['concat', 'uglify']);
  };
  </code></pre>
  
  <p>But I have this message :</p>
  
  <pre><code>Running "concat:js" (concat) task
  File "build/js/app.js" created.
  
  Running "uglify:build" (uglify) task
  &gt;&gt; Uglifying source "build/js/app.js" failed.
  Warning: Uglification failed. Use --force to continue.
  
  Aborted due to warnings.
  </code></pre>
  
  <p>Is there anything wrong with my config ?</p>
  
  <p>Thanks</p>