fix(package): update gulp-postcss to version 7.0.0

package.json

@@ -28,7 +28,7 @@
     "gulp-multi-dest": "~0.0.4",
     "gulp-notify": "~3.0.0",
     "gulp-plumber": "~1.1.0",
-    "gulp-postcss": "~6.4.0",
+    "gulp-postcss": "~7.0.0",
     "gulp-rename": "~1.2.2",
     "gulp-rimraf": "~0.2.1",
     "gulp-sass": "~3.1.0",