Don't use spread to increase backwards compat

tasks/build.js

@@ -44,8 +44,9 @@ function buildTask(options) {
               }
             })
             .on('end', function finishedTemplateDirWalk() {
-              const context = {
-                ...conf,
+              const context = Object.assign(
+                conf,
+                {
                   stylesheets: stylesheets
                   .filter(function filterFiles (file) {
                     /* Read only CSS files. */
@@ -56,6 +57,7 @@ function buildTask(options) {
                     return (prev += '<link rel="stylesheet" href="' + cssPath + '">');
                   }, '')
                 }
+              );
 
               options
                 .src([cwd + '/**/*.html', '!' + cwd + '/**/*.inc.html'])