export TreeTransformer (#1950)

- link to existing documentation on `TreeWalker` & `TreeTransformer`
- fix Travis build failures

fixes #1949

tools/exports.js

@@ -1,4 +1,5 @@
 exports["Dictionary"] = Dictionary;
 exports["TreeWalker"] = TreeWalker;
+exports["TreeTransformer"] = TreeTransformer;
 exports["minify"] = minify;
 exports["_push_uniq"] = push_uniq;