feat: use Yarn workspaces

Much faster installation which is very important for CI.

We'll see if it proves difficult for deps management.

packages/xo-server-load-balancer/package.json

@@ -32,12 +32,12 @@
   "dependencies": {
     "babel-runtime": "^6.11.6",
     "cron": "^1.1.0",
-    "event-to-promise": "^0.7.0",
+    "event-to-promise": "^0.8.0",
     "lodash": "^4.16.2"
   },
   "devDependencies": {
     "babel-cli": "^6.24.1",
-    "babel-eslint": "^7.2.3",
+    "babel-eslint": "^8.0.1",
     "babel-plugin-lodash": "^3.2.9",
     "babel-plugin-transform-runtime": "^6.23.0",
     "babel-preset-es2015": "^6.24.1",