build: transpile the debug dependency with Babel
build: transpile the debug dependency with Babel

https://github.com/socketio/engine.io/commit/a967626a1d1ec990090a63b4999337fd83abf223 has broken
the build for IE and android 5.1

Related: https://github.com/visionmedia/debug/issues/668

socketio/engine.io-client,Automattic/engine.io-client,socketio/engine.io-client,socketio/engine.io-client