remove start and end props from nodes in traverse.removeProperties

lib/6to5/traverse/index.js

@@ -77,6 +77,8 @@ traverse.removeProperties = function (tree) {
   var clear = function (node) {
     delete node.tokens;
     delete node.range;
+    delete node.start;
+    delete node.end;
     delete node.loc;
   };