grunt: use '' instead of "" as string delimiter

grunt.js

@@ -154,12 +154,12 @@ module.exports = function(grunt) {
   grunt.registerMultiTask('css2js', 'Embed CSS into JS', function() {
     var cssContent = grunt.task.directive(grunt.file.expandFiles(this.data.src)[0], grunt.file.read);
     var content =
-      "var css='"+cssContent+"';" +
-      "var cssTag = document.createElement('link');" +
-      "document.head = document.head || document.getElementsByTagName('head')[0];" +
-      "cssTag.href = 'data:text/css,'+css;" +
-      "cssTag.rel = 'stylesheet';" +
-      "document.head.appendChild(cssTag);";
+      'var css="' + cssContent + '";' +
+      'var cssTag = document.createElement("link");' +
+      'document.head = document.head || document.getElementsByTagName("head")[0];' +
+      'cssTag.href = "data:text/css,"+css;' +
+      'cssTag.rel = "stylesheet";' +
+      'document.head.appendChild(cssTag);';
 
     grunt.file.write(this.data.dest, content);