Fix how options are sent to asset SHA'ing

More detail on:

https://www.npmjs.com/package/gulp-sha256-filename#filename-custom-format

gulpfile.js

@@ -28,7 +28,7 @@ const paths = {
   assetsUrl: '/alerts/assets/'
 };
 
-const filenameFormat = '{name}-{hash:8}{ext}'
+const hashOptions = { format: '{name}-{hash:8}{ext}' }
 
 plugins.sass.compiler = require('sass');
 
@@ -43,20 +43,20 @@ const copy = {
     images: () => {
       return src(paths.govuk_frontend + 'govuk/assets/images/**/*')
         .pipe(dest(paths.dist + 'images/'))
-        .pipe(plugins.hash(filenameFormat))
+        .pipe(plugins.hash(hashOptions))
         .pipe(dest(paths.dist + 'images/'));
     }
   },
   html5shiv: () => {
     return src(paths.node_modules + 'html5shiv/dist/*.min.js')
       .pipe(dest(paths.dist + 'javascripts/vendor/html5shiv/'))
-      .pipe(plugins.hash(filenameFormat))
+      .pipe(plugins.hash(hashOptions))
       .pipe(dest(paths.dist + 'javascripts/vendor/html5shiv/'));
   },
   images: () => {
       return src(paths.src + 'images/**/*')
         .pipe(dest(paths.dist + 'images/'))
-        .pipe(plugins.hash(filenameFormat))
+        .pipe(plugins.hash(hashOptions))
         .pipe(dest(paths.dist + 'images/'));
   }
 };
@@ -214,7 +214,7 @@ const scss = {
           plugins.postcss([require('oldie')()])
         ))
       .pipe(dest(paths.dist + 'stylesheets/'))
-      .pipe(plugins.hash())
+      .pipe(plugins.hash(hashOptions))
       .pipe(dest(paths.dist + 'stylesheets/'));
   }
 };