Upgrade socket.io-client

This patch upgrades socket.io-client to v2. There's no breaking changes for us.

socket.io-client@2.x is still massive, but a bit smaller than v1:

```bash
> npm install socket.io-client@1
> echo 'require("socket.io-client")' | browserify - | uglifyjs -cm | gzip | wc -c | pretty-bytes
23.3 kB
```

```bash
> npm install socket.io-client@2
> echo 'require("socket.io-client")' | browserify - | uglifyjs -cm | gzip | wc -c | pretty-bytes
19.2 kB
```

package.json

@@ -80,7 +80,7 @@
     "namespace-emitter": "1.0.0",
     "on-load": "3.2.0",
     "prettier-bytes": "1.0.3",
-    "socket.io-client": "^1.7.3",
+    "socket.io-client": "^2.0.1",
     "tus-js-client": "1.4.3",
     "url-parse": "^1.1.8",
     "whatwg-fetch": "1.0.0",