let is not supported on Safari

supported browsers reference: http://caniuse.com/#search=let

src/definitions/modules/dropdown.js

@@ -3154,7 +3154,7 @@ $.fn.dropdown = function(parameters) {
 
         escape: {
           value: function(value) {
-            let
+            var
               multipleValues = $.isArray(value),
               stringValue    = (typeof value === 'string'),
               isUnparsable   = (!stringValue && !multipleValues),

src/definitions/modules/popup.js

@@ -189,7 +189,7 @@ $.fn.popup = function(parameters) {
             }
           },
           hideGracefully: function(event) {
-            let
+            var
               $target = $(event.target),
               isInDOM = $.contains(document.documentElement, event.target),
               inPopup = ($target.closest(selector.popup).length > 0)

src/definitions/modules/progress.js

@@ -601,7 +601,7 @@ $.fn.progress = function(parameters) {
 
         update: {
           toNextValue: function() {
-            let
+            var
               nextValue = module.nextValue
             ;
             if(nextValue) {