Do not use ES6 syntax since running Node 5 on Travis

Author: simeg

tests/datetime-spec.js

@@ -526,7 +526,7 @@ describe( 'Datetime', function() {
 	});
 
 	it( 'onBlur', function() {
-		let onBlurCalled = false;
+		var onBlurCalled = false;
 		createDatetime({ value: date, onBlur: function( selected ) {
 			// TODO: This is never called
 			onBlurCalled = true;