fix reducer that computes mtime for combined CSS file

Author: mojavelinux

gulp.d/tasks/build.js

@@ -32,7 +32,7 @@ module.exports = (src, dest, preview) => () => {
           .reduce((accum, { file: depPath, type }) => (type === 'dependency' ? accum.concat(depPath) : accum), [])
           .map((importedPath) => fsp.stat(importedPath).then(({ mtime }) => mtime))
       ).then((mtimes) => {
-        const newestMtime = mtimes.reduce((max, curr) => (!max || curr > max ? curr : max))
+        const newestMtime = mtimes.reduce((max, curr) => (!max || curr > max ? curr : max), file.stat.mtime)
         if (newestMtime > file.stat.mtime) file.stat.mtimeMs = +(file.stat.mtime = newestMtime)
       }),
     postcssUrl([