Include jquery.flot.stack* for using in multiple panels

When using multiple panels that use the stacked graphs
(e.g. histogram and multifieldhistogram) we need to inform
grunt-requirejs plugin that jquery.flot.stack* is one the
modules to prevent double-inclusion and then problems
in the histogram rendering.

See elastic#1296

NOTE: this change is needed for getting pull#1296 to work
properly but can be merged indipendently from it.

tasks/options/requirejs.js

@@ -53,6 +53,8 @@ module.exports = function(config,grunt) {
         'lodash',
         'filters/all',
         'jquery.flot',
+        'jquery.flot.stack',
+        'jquery.flot.stackpercent',
         'services/all',
         'angular-strap',
         'directives/all',