Improved grunt express/casper watchers

closes #9718

- fs.fileWatch that is used internally by 'gase' in 'grunt-contrib-watch', is having 100ms pooling default (07828a6845/lib/gaze.js (L36)). This is causing hight CPU usage for large amount of files.
- As suggested in https://github.com/gruntjs/grunt-contrib-watch#why-is-the-watch-devouring-all-my-memorycpu the watch interval was set to higher 500ms because the recommended default of 5s (https://github.com/gruntjs/grunt-contrib-watch/issues/145#issuecomment-20526067) was visible in the development flow

Gruntfile.js

@@ -70,7 +70,8 @@ const configureGrunt = function (grunt) {
                     'content/themes/casper/assets/js/*.js'
                 ],
                 options: {
-                    livereload: true
+                    livereload: true,
+                    interval: 500
                 }
             },
             express: {
@@ -84,7 +85,8 @@ const configureGrunt = function (grunt) {
                 tasks: ['express:dev'],
                 options: {
                     spawn: false,
-                    livereload: true
+                    livereload: true,
+                    interval: 500
                 }
             }
         },