chore: downgrade the debug dependency

The newer versions need to be transpiled to ES5, so we'll rollback for
now.

We'll have to find a way to provide the users who bundle the Engine.IO
(or Socket.IO) client (with webpack for example) with a source code
which does not include debug (or document the usage of
webpack-remove-debug [1]).

Reverts 242ea9e

[1] https://github.com/johngodley/webpack-remove-debug

Author: darrachequesne

package-lock.json

@@ -26,7 +26,7 @@
   "dependencies": {
     "component-emitter": "~1.3.0",
     "component-inherit": "0.0.3",
-    "debug": "~4.1.0",
+    "debug": "~3.1.0",
     "engine.io-parser": "~2.2.0",
     "has-cors": "1.1.0",
     "indexof": "0.0.1",