transloadit: pin socket.io-client to ES5 version

packages/@uppy/transloadit/package.json

@@ -32,7 +32,7 @@
     "@uppy/tus": "file:../tus",
     "@uppy/utils": "file:../utils",
     "component-emitter": "^1.2.1",
-    "socket.io-client": "^2.1.1"
+    "socket.io-client": "~2.2.0"
   },
   "peerDependencies": {
     "@uppy/core": "^1.0.0"