phantomJS requires "use strict" for polyfill to properly work

app/assets/javascripts/extensions/array.js.es6

-/* eslint-disable no-extend-native, func-names, space-before-function-paren, semi, space-infix-ops, max-len */
+/* eslint-disable no-extend-native, func-names, space-before-function-paren, semi, space-infix-ops, strict, max-len */
+
+'use strict';
+
 Array.prototype.first = function() {
   return this[0];
 }